Power Expander problems, please help!

Hello! Recently we’ve used the Vex power expander to have two batteries, we’ve hooked up 6 motors (using 2 y-splitters) to the power expander. However after driving around for about 20-30 seconds, the motors stop working and seem like they are trying to go back and fourth, I’ve tried using a simpler code and I’ve tried checking to see if they’re getting hung up, but they aren’t and I have no idea whats wrong. I suspect that its the power expander because its only happening to the motors on the power expander.