Fluval filter lost pressure

Hi, I have a Fluval 2+ filter (one of the completely under-the-water types).
It's been running fine for a month or so but lately it keeps losing
pressure. Instead of firing a mighty jet of water it has reduced to a
trickle. I've "fixed" it in the past by unplugging, taking it apart and
reassembling but that doesn't seem to be working this time. It's not clogged
up or anything so I don't know why it should be running so weakly. I would
have thought the motor would either work or not work, but it just seems
erratic. Any ideas?

Thanks,
Dan