There are so many things to check that could cause the issue that it's hard to think of where to start. I've been working on one that I've already spent two hours on just to finally discover the pump isn't making enough pressure, so I'm thinking about converting it to a peristaltic pump like Fragramatics. Then maybe I can find a gallon jug that will fit behind the
door and I don't have to deal with the tank growing scum in it.
If you're getting plenty of liquid and air but it's not foaming, the wadding in the elbow behind the brush may have slipped out. I usually replace the foam pellet in the Fragramatics brushes with one cut out of an
Armor All sponge - you could try stuffing some in and holding it with a toothpick just to see if it foams.
I clean the reservoir by getting a pot of water almost boiling and pouring it in the tank and flushing it out. Bleach might work too, you just want to kill the algae that grows in it and flush it clean.