Trouble docking

I ran this vacuum for the first time yesterday with no problems. Today it seemed fine until it attempted to go back to the dock and charge, it had trouble climbing up the ramp to get into position. The wheels couldn't get traction or one of them would and then it would turn rather than back straight in.  I didn't see anything stuck on the bottom. Anyone seen this?

I had same problem. Fixed it by filing down both triangle bumps on the ramp. It docks 100% now.