Like other people have said, they do lose their siphon sometimes. I make it a point to always fill the filter with water before turning it back on. I have found that all my AC's that are smaller than the 110's seem to more problematic, they seem underpowered compared to the 110's.
sometimes have you to flick the impeller with your finger on the smaller AC's./
the gaps in the impeller housing are small enough that a bit of gunk/slime will cause the impeller not to spin when the electricity is applied to create the magnet that drives the impeller.
it happens on my smaller ones often, but never on the big AC110's. probably for two reasons: more current = more magnetic = more torque