To help assess your problems it would be good to know:
Are you using the Cortex controller or the older crystal set?
Are you using EasyC or RobotC?
Do you have a good connection between the controller and the robot? (Don't know?... check the LED's... see next question.) The robot will run autonomous code without a connection to the controller, but will not execute driver mode code without the controller.
What LEDs are showing (on the robot and controller) when you run into the problem?
Does the system work with the "default code"?
Got a camera or camera phone? Post a photo or two of your setup.
VEX has a pretty darn reliable system, so you're very likely to get it working. You may also want to cross post your question (with some of the data, above, attached) to
www.vexforum.com
Jason