"I've included x pages, which is more than you requested, because the first x pages are a prologue."
And every agent ever hangs head and sighs in frustration.
I've made it no secret that I'm not a fan of prologues. I can count on one hand the number of times I really liked a prologue or felt it added to the story. 99 times out of 100, the prologue turns me off to a story rather than grabs me in. I could argue against prologues based on my own feelings, but that's hardly useful. Here are some prologue types that probably aren't working.
1. The epic fantasy ballad
Some authors feel the need to start their story off with a poem, song, fiction text passage etc. I assume they feel this sets the tone of the book. It may do that, but for me the tone is "Zzzzzzz...". Confession time, folks. When reading for fun, I skip sections that include any of these items that are longer than a paragraph. I'll give you one paragraph to wow me with your characters' poetry skills and then I'm skimming to where the action starts again. This could be just me, but if we played a little game of truth, I'd be surprised if I was the only one skipping these parts. They just aren't interesting. Sometimes the truth hurts.
2. Inside the antagonists mind
Most books are told from the POV of the hero, and that's fine, but it can limit our knowledge of the bad guy. For some reason, authors feel like the prologue is a great place to show just how evil the antagonist is and why we should hate them. First, this is lazy. We should be able to get this information through the course of the novel. We don't need it dumped on us in the beginning. Second, I probably picked up this book because of an interest in the protagonist. So that's who I want to meet, not the bad guy and his evil minions.
3. A trip in the time machine
All of your characters should have a complex back story that makes them who they are in the present story. Some of that will come out during the story and some of it will just be part of a list that helps you craft a well-rounded character , but never makes it on to the page. What I don't need is a flashback showing me your character ten years ago that explains why he/she is the person they are today. Again, this is lazy writing. I don't need to understand from the get go that your retired cop hates poker players because one swindled him out of his retirement. Let me learn about his hatred when his daughter introduces her new boyfriend, the rising Vegas star. Hint to his reasons during their discussions and let the revelation come out when dad has to convince the boyfriend his daughter is worth the hassle of having a crotchety father-in-law. Trust your reader and make them work for the big reveal.
4.Random info dump
Probably my least favorite prologue is one featuring characters who are neither the hero or the bad guy. This is usually done with characters close to the MC, such as their mother, and reveals a secret that is hidden from your protagonist. While this can help a reader understand the motivations of secondary characters, it can also lead to frustration. Now we have information the main character doesn't. His actions will be judged based on this information and we'll be constantly reminded we know something that he doesn't. I find myself screaming at the page for the main character to hurry up and "get a clue" already. This isn't going to lead to a good reading experience.
5. Floating on the wind
This is the prologue where the readers is flown over some fictional town as if a leaf floating on a warm air current while an omniscient narrator tells us all about the gossiping biddies in the beauty shop and the overweight men pretending not to check out the nuddie mags down at the drug store. As a reader we are expected to either feel comfortable with the familiarity of this fictional town or disturbed by its different-ness. Either way, I don't care. I don't read a story because of the city it's set in.
Here's the real deal. I ask for five pages as part of the submission process. That's pretty generous as most readers won't go past the first page if they aren't pulled in. If you feel the need to include extra pages, it's probably because you don't think I'll be pulled in by the prologue. If I'm not, what makes you think editors or readers will be. If you have doubts about your prologues ability to pull in a reader, the answer isn't sending more pages. It's giving your prologue the axe.