Before you go buying crazy parts or junking the filter all together, fill your tank and/or clean all the parts. To prime it, while it is running, pour water into the pump chamber over where the intake pipe covers the impeller.
Often, when the unit has been in use for a while, it will need a good cleaning. The impeller housing gets gunk growing in it and needs to be cleaned now and then. Remove the system from the tank and take it to the kitchen sink. Run a pipe cleaner or small bottle brush in the housing to loosen the crud in there. Then turn it upside down and spray it out with your kitchen sprayer.
The intake tube also grows crud inside of it to the point that it blocks the water flow. I have a flexible sink trap brush that I use to clean mine. You can also get most of it out by taking it outside and using a garden hose to force water through it.