Try to stick it in with a thin wire or needle first. If it doesn't work or the bubble is very small, see if there are any screws on the bubble pressure plate. If there is, it can be removed. (of course, it won't leak if it is removed! ) Have a look after cleaning.

If there are still no bubbles, it's the air pump. If you want to have the best of both worlds, you have to go to the local after-sales service!