Is it the soft rubber style bubbler? I use these, and sometimes mine stop too. To get them bubbling again I put it in a bucket of old tank water and rub it down. Algae and stuff will block the pores and make it hard for air to escape. Just give it a light rubdown every time you do a water change and it will work. You don't necessarily need to take it out every time, but if it's clogged now, pull it out, and just rub it off in the tank during your weekly water changes afterward.