MakerBot Industries

I think three of my original printed pulleys were like that. One I reprinted and replaced very early. The second I used (home made) ABS glue on to fix. The third wobbled on its bearing until it wore the hole larger and the pulley fell off the bearing, which made the belt slip off. (I had just printed its replacement luckily — otherwise I would have just used ABS glue on it too.)

I'm certainly no expert here, but if the pulley wobbles on its bearing, you need to do something, either by replacing it outright or gluing it in.

ABS glue can be made by dissolving (soaking until not solid, really) some scrap ABS in a bit of acetone, and then add more scraps until it is thick enough. You probably need a sealed jar, otherwise the acetone evaporates faster than the plastic dissolves.