I am in edit mode, which means I'm thinking a lot about writing structure. Here's the deal, I'm about to list a bunch of things, they are not rules, because you can find an example of a great story that breaks every rule.
And, not to be all you haven't seen what I have seen, for the reader folks among you, let me tell you - if you've done contest reading, slush pile reading, or anything where you are exposed to a bunch of unpublished possibly newbie writers, there are so many more prologues out there. So, if your sample of prologues is published prologues, consider those Olympic caliber prologues.
1. I believe it was Jennifer Crusie I heard say readers are like baby ducks, looking to imprint.
If the first character is not a main character, it can be disorienting. You spend all this time with this character and wait, they are the villain? They are dead now? They are just introducing us to magic? Now I have to learn about their great great grandkid?
2. Similarly if the setting is different, the reader may be very excited to hang out in ancient Rome, except in the next chapter we're in China and now they need to adjust.
3. And if the time period shift - anywhere from 6 days to six millennia, you are leaping your reader to a new place. Yes chapter 1 and chapter 2 can often be from different POVs or locales. But each switch requires resetting on the part of your reader.
4. If the prologue is there because the prologue is more interesting than the chapter that follows it, this is a tricky thing to pull off. In some circumstances it works great. It's so cool. But you also run the risk of losing the reader because if the sample that got them excited was full of intrigue, but the mood of the story is different, well, maybe I didn't want that. And the length of time I will care to get back to the place you promised me is limited.
5. If the prologue is the same character but at a different age, there are two issues I see. One is a craft issue. Not everyone is as adept at writing six year olds as they are adults.
6. The other issue is not every reader enjoys six year old characters, teen characters, 400 year old characters, so they may opt out, thinking this wasn't the story they wanted.
7. And yes, I know I wrote 7 things, and then gave you six that boils down to, if your prologue doesn't match the rest of your story, you are potentially confusing the audience. And look, readers are smart. I see tips out there like call your prologue chapter 1. I have read books that did this, and I still knew, once I turned to chapter 2, and saw we were changing to a new POV, that I had read a prologue.
So what are my fixes? First of all, as I said some stories do all these things I complained about and are still great. Some stories are actually better told in an unusual order. (Though I think it's worth noting, if your story isn't good in chronological order, jiggering the timeline only disguises so much.)
I confess, I have a huge peeve about extra POVs. I'm not against an ensemble cast, but gosh, if all your extra POVs die, then so does my trust in your extra POVs. But, there are plenty of books I've read that do it and make me care and make me feel so fast that I go with it.
It's a little bit like the length argument. I won't care if your movie is three hours long if it's three amazing hours. I won't care that you had a prologue, or disguised your prologue as chapter 1 if it's a great prologue that enhances the story.
But sometimes, the prologue is for the author, so the author can get into the zone, the background, or foreground of the story. And in those cases, it should be snipped.