I'm teaching a summer program using the Rokit kits and some of my students with MacBook Pros (with, I assume, Mac OSX) are unable to upload code, instead getting a message that the programmer is not responding.

We've followed the instructions for installing Arduino, downloading the Rokit files, and installing the CP210x driver - which other students on Windows and different models of Mac have done successfully. We've also double checked that the boards are connected and in programming mode and the right board and port have been selected. None of these issues seem to be responsible for the error as far as I can tell.
Any help resolving this would be greatly appreciated, thank you.