I set up the Eggbot with my daughter for another session of printing and she immediately started hitting problems. This is what we observed:
The pen would flop to the right and the drawing would all clump up at that side. Starting with the pen swung to the left to compensate and the drawing would clump to the left.
Pen rotation was very jerky.
There were a couple of red lights on the board at the left side with the connector panel for the small wires.
Pen arm rotation seemed to always be to one side when done manually for a number of steps. Positive seemed to be under control, negative flopped more in same direction.
This seemed to align with wiki manual instructions about adjusting the potentiometer for the motors, but that didn't seem to help. It also did not mention the red lights.
Eventually I looked at the physical wiring connections of the motors and found the brown wire at the bottom disconnected, which is one of the wires to the pen arm. Reconnect the brown wire and the Eggbot went back to work like normal.