Or: Bob's Event Horizon
The comments: "This is backstory." or "This is where I stopped reading." Or "Cut these first 2 paragraphs/ 2 pages/ 2 thousand words/ 2 chapters. They're boring/ really boring/ eye-gougingly boring."
What the comments can mean: You've started your story waaaay before anything significant to the plot actually happens.
If you're saying "Nu-uh. I totes started my awesome epic tome on the night Bob the Grandmaster mage was born." Cool. Is the story about his early childhood? His formative years? "Nah, it's totes about how an evil mage takes over his village and kills his parents and he has to avenge them, thirty years later. Oh wait, should I start when his parents die?" Uh… "Or, should I start with a five-chapter description of how he trains to become uber powerful?"
The short answer is no. Did George R. R. Martin's series start with Ned Stark's birth? Or his marriage to Catelyn? Did the Harry Potter books start with the day he was born? Did Hamlet's story start with two scenes showing his father's murder? (The answer to all four is no.)
Stories start the moment the main character's life changes – as it pertains to *what* the main story is about.
When you first decided to write the full story – I'm not talking about how you imagined Bob would look like, or describing Bob learning his first spell – I'm talking about that a-ha moment, that epiphany, that heart-quickening, mouth-watering, head-banging-on-desk moment when you shook your fist at the sky – what was the story about? What the f*ck went through your head when you saw Bob's world – not his parents' world, not the village's world, Bob's world – turn bat poop cray cray? Usually, it's the moment when *drumroll* nothing will ever be the same again. Seriously, it's the event horizon of your plot, the moment when there is no turning back, no regaining equilibrium, no return to your world's normalcy. Because up til Ned agrees to leave Winterfell, his world could have returned to normal. Up til Harry meets Hagrid, he could have stayed with his aunt. Up til Frodo decides to leave the shire, he could have stayed comfortable in his hole. Events in their respective worlds would have unfolded, but without these characters. The stories would be about someone else, eh?
Find your character's point of no return. That is the first scene of your story.
Let's put our serious pants on:
Maybe you're sure, beyond all doubt, that you've begun your story at the absolute perfect point. You might still get those comments because your message – the beginning of the story – is unclear. Sure, you may have skipped Bob's birth, childhood, and training years, but did you describe them? Did you include a mini-handbook on how magic works in Bob's world? Did you devote pages and pages to flashbacks describing what his parents used to look like or the topography of his village before and after the massacre and the paragraphs look like "Bob remembered his mother's blood-curdling scream as the black-robed nefarious Archmage of the Twelve Hells, Rizzo, flooded the village with streams of orange and red balls of fiery flaming spheres"?
Or, did you literally drop Bob in the middle of a fight scene that looks like "Bob ducked as a fireball whizzed overhead. It struck a gomba, consuming it in magical purple flames. Bob brushed a tear away, sad for the gomba, and vowed to avenge it as he'd vowed to avenge his parents. Lord Rizzo would suffer in the ninth level of the twelve hells, because the ninth level is where humans go, unless they died between the ages of nineteen and thirty-five. Then they go to the eleventh level. Except for Lord Momo. He got his own level, according to Sir Beemo's treatise on The Levels of Hell for Dummies. But Bob didn't believe in any of that stuff, really. He only wanted… what was it? Oh yeah, vengeance."
Then you kind of started at the beginning, and kind of didn't. The story got buried. Try cutting everything that the reader doesn't need to know *at that moment* in order to understand the scene. Take advantage of a reader's suspension of disbelief and abeyance to move your story forward, always forward (even when using a flashback!). The caveat is there's a time limit. If the comment was "This is all backstory," and your reaction is "No it's not, it's significant later on," then you have missed the cutoff point for the reader's abeyance. In other words, you've either waited too long to reveal the connection, or the connection is not clear enough. And unless you have a magic spell, miracle whole-body lube, or steam-powered giant hamster ball that allows you to slip between the laws of physics, you won't be behind every reader, whispering "Keep reading – it gets better. I swear!" That part you want them to get to? That's probably the start of your story.