I don't know anything about that filter, but I can suggest a few things.
Did you clean the impeller? Sometimes the impeller can get clogged, and it's easy to miss when you do a filter clean.
Did you put the filter back together properly? If one little bit isn't in quite the right place, it can stop the filter in its tracks.
If it comes to getting a new filter, for a 40 gallon tank I would recommend a Fluval 404 or 405. It'll more than get the job done in the current tank, and when it comes time to upgrade the tank, you probably won't have to buy a new filter as well.
Just remember, you want something that pumps at least 3x the gph as the size of your tank (so you'll want something rated at least 120 gph). The higher the better though, and there's really no such thing as too much filter