Converting a toy from battery to DC power adapter
Assuming that your AA batteries are alkaline cells, the bubble machine is pulling about 1 amp. AA cells are (roughly, it depends on current levels) about 1 amp-hr capacity at 1 amp, so 1 amps times 1 hour is about right. 

Take a close look at your AC adapter. It will probably say 12VDC and some number of amps. In this case, it's probably in the 1 to 2 amp range, and closer to 1 amp. If this is so, the adapter is unable to produce 12 volts at the current required. This results in weak motor drive and sounds about right for your problem. 

The only solution, if I'm right about the adapter, is a different adapter with a higher current rating and the same voltage rating.