As stated already could be the membrane jamming up things or it also could be a bad electric pump motor...after running a bit 'bearings' start to lock up or armature overheats.
In my case it was the membrane bunging things up (Membrane detached from Plunger) >>>
All the pieces from the membrane>>
Once you get apart here are your basic components>>>>
If it is a membrane issue then you can repair>>
If it is the motor unit... well then you will be getting a replacement pump which runs about $150 new (the OEM manufacturer no longer makes the lumbar pumps)