One friend of mine told me that the scenes in my mystery novel-to-be go abruptly from, for example, a scene at the main character's home, and to the middle of his conversation with the detective. The friend also found it strange that the main character remembered the detective from case X, with us not being told about that case earlier.
How should I go from scene A to B exactly? I had been using an ellipsis after being satisfied with the scene as it was.
It is difficult to assess whether your friend's critique is adequate or not without reading the respective passage, so my reply will be general and not specific to your case:
1.
When you want to switch from a scene at the main character's home to a conversation with a detective elsewhere, the transition is more smooth if the two scenes "refer to each other" somehow. For example, at the end of the home scene you could mention that the main character now plans to go see the detective, and at the beginning of the conversation scene you could summarize in one short subordinate clause how the main character had got there:
[home scene] ... John decided that he needed to see Detective Peterson.
* * *
After trudging through the rainy streets for an hour, John was in no mood for his friend's usual banter. "Listen, Peterson", he interrupted the detective, ... [conversation scene]
You should introduce information that leads to a plot turn before that turn, otherwise it will, for the reader, come out of nowhere and appear random. For example, if your main character can do Kung Fu, you should mention that long before he suddenly does some moves to incapacitate a much stronger opponent.
Backstory, like the fact that your main character is familiar with the detective he is talking to, is usually introduced when it becomes relevant. That the two are already familiar and not meeting for the first time, is immediately relevant when you introduce the detective to the story, so you should mention it immediately.
Using my previous example, there are two places where you could mention that your main character already knows the detective, either when he first thinks of him, or when he meets him (I've emphasized the changes to make them more apparent; you wouldn't of course emphasize them in your text):
Variant (a.1):
[home scene] ... John decided that he needed to see his old friend Detective Peterson.
* * *
After trudging through the rainy streets for an hour, John was in no mood for his friend's usual banter. "Listen, Peterson", he interrupted the detective, ... [conversation scene]
"Old friend" at the end of the first scene is completely sufficient to tell the reader that the two have a common history. You can then mention any part of that common history, like a case they have been working on together, whenever you want. The reader will not be surprised that a detective will have been working on a case, so that information will not be a surprise when the reader already knows that the two characters are long time friends.
If what they experienced together is uncommon and cannot be induced from the one person being a detective and the two being familiar alone (e.g. the detective had to shoot the main character's criminal wife), you should mention that immediately:
Variant (a.2):
[home scene] ... John decided that he needed to see Detective Peterson, who had shot his wife.
* * *
After trudging through the rainy streets for an hour, John was in no mood for his old enemy's usual banter. "Listen, Peterson", he interrupted the detective, ... [conversation scene]
You can still provide the rest of the backstory later, but in this case you must provide it eventually. If the two are merely friends, you don't have to give any details of their past at all.
Variant (b):
[home scene] ... John decided that he needed to see Detective Peterson.
* * *
After trudging through the rainy streets for an hour, John was in no mood for his friend's usual banter. He had met the police officer while he had been working as a coroner in Davis county, and the two of them had solved a series of gang related murders. "Listen, Peterson", he interrupted the detective, ... [conversation scene]
Whether or not you'll want to provide additional details for the backstory will depend on the relation between the backstory and the present story.
Feedback from readers is challenging. Especially when it's couched as specific corrective actions -- you need to xxx or yyy. Readers who can express their feedback in terms of their own reactions are very valuable people to listen to.
For the rest, I've found it's important to work backwards from the 'You need to do this' kind of statement and infer what they are reacting to in my story.
In your example, I'd imagine the readers are not getting a strong sense of setting in either scene, so when the scenes shift to a new setting they feel removed from the story.
As for how should you go from scene to scene, do what you think is right for the story. Examine books you've liked and see how those authors moved from scene to scene. Sometimes it's very abrupt and sometimes there's a lot of detail about the passage of time or change in setting.
Personally, I try and put my scene breaks at either moments of highest tension or at natural pauses, when the conflict of the scene is resolved.
A side note: I assume you mean you've been using an ellipsis to mark your scene breaks.
Standard manuscript format uses either three asterisks (***), three hash marks (###), or a single hash mark (#) to mark a scene break. If you are not using the standard format, you can do whatever you want
Abrupt scene transitions can be made more palatable by setting descriptions. For instance, when you are switching your main character from his home to his conversation with the detective, you might add something like: His phone rang while he was driving to work. It was Detective XYZ.
I don't find it odd at all that the main character remembered the detective from case X, but it might be helpful to add a short description of the case, and/or why he remembered. For example, "I remember seeing you on the news after you cracked the Wells Fargo Bank heist last year."
