try unplugging it and checking for air escaping into the outlet tube, if there is air, then plug it in till its gone, and unplug again and replug etc etc
until you can't see any air building up in the output tube after you unplug it
or, alternatively, you can unlatch one latch while its running, lift on the lid a bit until you see the water inside getting near the top and then relatch it.
I've found this most often happens when you A. don't wait long enough for the filter to prime after cleaning it, and B. In winter, because of the high amount of gases in the water, after a water change the escaped gases can cause it to air lock if you are using a spray bar vertical or your jet output is deep in the tank
