Too Much Code??

[Exernie]

When i write the code, for example a 4 wheel drive system and i build and run it works through the driver station. but when i add to the code, say for example a motor, nothing works. but when i make a code only for that motor, it works. i just can't get them together. Help? could it be the classmate being stupid?
ps. the driver station says one loop could be starving another.