Fluval 304 - Low Flow

I have a Fluval 304 cannister filter that I am not sure is working properly. I have carbon running on the bottom layer and the middle has carbon and the bio-stuff and the top has the bio-stuff too. It seems that the flow out of the unit is failrly low. I would of expected a brisk stream but it is more like a fast trickle. I have tried cleaning the foam filter and this doesn't make a lot of difference. If I give the unit a few pumps from the primer it will speed the flow up for a day or so and then it slowly reduces the flow.