I've researched the problem a bit but cant seem to find anyone with a definitive answer.
I have my summit with a Tekin RX8 motor/esc combo on it.
When I give it throttle on the ground it does move forward until I put a little resistance on it. For example if I nudge it up against a small curb, it will not go over that curb. It will however drive on the bare pavement with no problem.
It makes a squealing type sound when this is happening. I've checked the spur gear > pinion gear at that makes contact it seems without a problem.