If the water level is high, no cracks or leaks, then wiggling the tube from high intake to the side for lower intake for a few seconds should do it. Try taking off all other tubes other than just the U, keep fish away from the intake while the end/screened tube is off. It should work just fine.
I have two 110s and after a power outage, I have to move the U intake back n forth.
Try filling the tank to the max, IE, all the way up to the inner black frame lip.