afremont:
The stall current of the motors is such that they would overheat the controller pretty fast, but I assume that you aren't just turning them on at 100% duty cycle from a stand-still, are you? What PWM frequency are you using, 10kHz is pretty high. Depending upon your motors, they might have too much inductance to actually draw much power at that frequency.
HazardsMind:
Your motors probably dont have enough torque, in which case you will some kind of gear system.
JimboZA:
It might be a simple mechanical problem to do with (lack of) gearing.Have you got a motor driver chip in the mix somewhere though?- if you have say a 298 in there you'll be losing volts at the best of times but even more if the current is high with a motor trying to get the robot started. What's the voltage at the motors?
I'm using these motors,
http://www.parallax.com/StoreSearchResults/tabid/768/List/0/SortField/4/ProductID/587/Default.aspx?txtSearch=motor+wheel+kit
and this motor driver