Sounds like a priming/build-up issue.. clean it... take out the intake tube, and the impeller houseing.. use an old toothbrush and clean it all with just water, rinse it well ect. fill it up... the back chambers take abit of water.. so fill it up until the return starts to flow back into your tank.. then start it up... if its the older model with the flow control pull it all the way up until it clears the air then adjust it how you want. I'de also run the toothbrush over the spraybar return holes.. those get gunked up on me all the time... I love my 4 Emperors 400's and 1 280... I hate the one AC i have ( its in storage now) and the Aqueon i recent got (always gets clogged up and it's rated for a 40.. not the 20 it's on) they do splash abit.. but for me the spot on water conditions they maintain are worth it.
If you're still haveing issues you may need to replace the impeller. I have had to replace my impellers on my penguins before when a small rock gits into one and destroyed the shaft. when you clean it make sure to clean any white build-up off and check the impeller shaft best you can to make sure it's not deteriorating. I just use warm water, and elbow grease with a clean old toothbrush.