Orbital sander no longer spins when base is attached

Hi- I have an orbital sander that I bought in a pinch a while back, it's a rather mediocre house brand from Canadian Tire (in Canada) called Maximum. Anyway, it was seizing up and acting weird, so I took it apart and blew it out with air pressure. Before putting the base back on, I tried it and it spun great, sounded great again. But with the base back on it seizes. The base doesn't seem deformed from what I can see.

Ideas about this? Thanks.