I originally had PLA in the left extruder of my Dreamer and ABS in the right. A few days ago I unloaded the left, put in a different color ABS and ran a few prints. Now when I unload the left and try to put the PLA back in, it feels like it's grabbing the PLA, but then it just goes thunk-thunk-thunk and the PLA doesn't advance into the extruder. If I unload it again and load the ABS back in it works just fine.

Any thoughts?