This is most likely a PTC issue. Here is a detailed inspection of the PTCs found in Vex motors and Cortexes. Essentially what happens is that as motors are put under load, they tend to heat up. Eventually they dangerously hot, putting them at risk to permanent damage (melted plastic, burned out electronics, etc.). To prevent this, Vex motors have PTCs built in to them. When they get too hot, they will cut off power to the motor for ~10 seconds, depending on how long it takes to cool down. If there is too much friction or too little torque in your drive, the motors will overheat and trip the PTC. When this happens during a competition, it is best to turn off the affected motor(s) and wait 10 seconds before using them again. To actually fix the issue, you can do a number of things:
- reduce friction
- add torque (another motor or a slower gear ratio)
- don't supply as much power to the motor (multiply the joystick value by 0.6 or so)
Probably make sure the shaft is connected first, though. That's a much simpler fix.