No comm link messages

I am trying to have my students in POE class update firmware on several cortex microcontrollers.
We keep getting an error message that says no communications link is available, and we keep getting it regards of changes to platform type, turning cortex on/off, unplugging/changing USB cables, restarting program, etc.
Can someone tell me why, and the proper steps to take to avoid this problem.
I am starting with simple programs using VEX cortex, not using Vex-net at this time.

Thanks in advance

Do you have the cortex plugged with the USB to USB to the computer?

I have the communication set at USB only and platform type set to 2.0 Cortex. I have the USB cables plugged in to the controller and computer, and have replaced a few cables on some. I have replaced all the batteries with brand new 3000 mAhr ones that were on the chargers all night. I have learned they work best when the controller is off then USB connected, then power up, but it is not resolving the issues. Even today, I still had about 7 of the 12 teams unable to upload their first program, despite trying several times to shut down, and start up as described. The firmware, master and RobotC was updated literally minutes before the class started.
I have seen this problem before, but this is the first time I am trying to teach a large group, and it is getting frustrating.
Thanks so far.