Trackpads are finicky - I don't know of anything that you could do short of running an Apple Hardware Test and, even is all hardware passes, take the machine into your local Apple Store or AASP.
Sounds like you possibly have a faulty tracker pad. Quite common on these 13" Macbook Unibodys. Iv changed quite a few of them for customers. Tell Me! When you click down on the tracker pad does it feel like its responding as it always has or does it feel like kind of sticky?
If it still feels like its clicking correctly but not actually working its more likly a faulty trackerpad. If this is still under warranty I would take it in to Apple. You won't be able to do anything about it yourself as the battery is held in place with a torx screwdriver only Apple have access to.
In conclusion if all hardware tweeks have not solved, its time to take a trip to an Apple store.
As your Macbook is not that old I really cant see it being dirt.