Hi Johnny, you could also have air trapped in the cannister, tip the filter to one side carefully and see if it pumps out air.
As with all impellers it may be that the impeller needs packing out on the end of the shaft that is furthest away from the impeller blades, there are available small nylon washers for the job, if this does not cure it you may have to buy a new impeller. 8)
Doh, I use a small baby bottle brush to clean the impeller housing, this gets gunged up with bacteria and detrius. 8)