Sorry I've not been on line for a few months so haven't seen your post until just now..
I've had the exact same problem with the U1. I found regular removal and squeezing out of the sponge works.. For me it was a build up of crud within the sponge causing the issue. So when I did my weekly PWC
I'd take the sponge out and give it a quick squeeze in the bucket of waste water, the flip it over and pop it back in.
I ended up buying a load of biorings from eBay and using them as well as the sponge in my U1. I cut a vertical slice about 1cm thick from the middle of the sponge, where the apex of the arch is. Then dropped a load of biorings in the resulting gap.. This gave course mechanical scrubbing on the inlet (sponges), and lose packed biomedia directly above the pump inlet so it doesn't get clogged any more...