Printer responds only when powered by USB (no replies)

I have been trying to test the electronics for a reprap I'm building, but can't get the motors to move. I have removed the D1 diode on the RAMPS so I can power it with a PSU while running the Arduino from USB. The PSU I am using is an old laptop brick that I converted using this guide. I have Marlin running on the Arduino, and can connect to the printer with Pronterface and it will display the firmware version and name. However, when I try to move the motors, nothing happens. If I try to send the M119 command, it just says "SENDING:M119", but then nothing comes back. When I turn off the PSU and connect to the printer, I can send M119 and it will return the correct endstop statuses. Basically, my computer can only communicate with the printer when the PSU is off and the whole thing is only receiving power from the USB. I have also found that I can't upload sketches to the Arduino when the RAMPS is attached.
Configuration.h
Board wiring