I had this problem for weeks with the same filter.
Clean the propellor compartment out just in case.
BUT the problem was the sponge was too dense for the gunk to get through it and it was blocking that little hole up at the bottom. Every 5-6 days I'd have to restart my filter and clean it...but since adding a few ceramic rings to the bottom of the sponge compartment I've only had reduced flow once (in about 2+ months). You just need something to lift the sponge off that hole. I've since bought a second U1 filter for another tank and straight away I added ceramic rings and it's been filtering away nicely since I put it in.
Plus when I cleaned the filter after 2+ months you could see most of the sponge was dirty as opposed to just a tiny bit at the bottom previously.