I have a volcano exp pro brushless that just started a problem when you try and take off it chatters a few times and wont move,sometimes you can give it a little shove and it will run fine until you stop then chatters and wont move again.If you hold it up off the ground it will run just fine.
I changed the spur and motor gear and looked at the rear gears they look good,but still wont move???????????

We had similar issues as the truck wouldn't start on an incline. Redcat sent out a free replacement ESC. We havent installed it yet but they said it was the likely cause. You can also check the ESC program to make sure the 'punch' setting is not the problem.