Could be the angle of attack, could be the energy you're applying, could be the tine quality, could be the supporting of the piece, could be a weak platform, could be the grain of the rock. Any of these variables (and more) can easily play a role in what's happening.
Show us the before and after edge you attempt to flake. Show us the resulting tiny chips.
Take a photo of how you are holding the piece you're working on. Are you bracing on the inside of your legs?
Give as much info as possible.