I have been querying an MG novel and have had several requests for the full manuscript. Unfortunately none of these have turned into offers. I've been lucky enough to get some feedback from the rejecting agents and almost all of them say something about the "voice" in my book.
Can you shed some light on what "voice" means? It's pretty hard to try and fix something when I'm not sure exactly what they mean.
"Voice" is a tricky one because it can refer to several different things. There's your own authorial voice which is to do with your style as a writer - your choice of words and the rhythm of your sentences etc. Then there is the voice of your novel which is more to do with your characters and the way they express themselves.
The two are obviously linked, but I feel like in this context the agents are more likely to be commenting on the voice of your novel. MG is especially tricky in this respect because the voice can't come across as too young, or as too old. Kids in that MG age-group have a unique way of expressing themselves and of seeing the world around them. They can also differ wildly in terms of their maturity, so it's often difficult to figure out exactly where to pitch your voice.
My suggestion would be to read widely in the MG sphere so you can get a good understanding of what MG voices sound like. Spending time with kids in this age-group would also beneficial. Kids often won't speak freely in front of an adult if they know they are being listened to, so I like to do it on the bus or at the mall or library. I put headphones on so they don't think I can hear them, then listen to the rhythm of their speech, the words they choose and the things they feel are worthy of conversation.
Once you have a good feel for what MG sounds like, you can go back to your MS and see if what you have written would fit authentically within their syntax and world view. Make sure conversations flow the way real kids talk to one another and that they way your characters react to situations is realistic. Kids can be pretty worldly, but their understanding of things may not yet be complete. Don't be afraid to let your characters get things wrong, to misunderstand things that would be obvious to an older person.
But also don't talk down to them. Nothing will turn off a young reader more than being condescended to and making your character act or talk too young will turn off readers as much as making them act or talk too old.
Hopefully this will help.
Good luck with any revisions you make and your on-going search for an agent.