Is there liquid in the air chamber?

You should see it inside the chamber below the pump. If so, turn it upside down and pump a few times which should get the water out of the air chamber. If that doesn't work (and if you are brave) take the pump apart, clean it completely, dry thoroughly and reassemble.

Has there liquid in the air chamber for a while?

If so, clean as above. However, if the soap has had time to degrade the silicone in the pump, it may not regain the original amount "spring" back up.

Is the screen clogged?

Empty the container, clean the screen and flush with water (it won't foam). Use a different liquid soap without particles in it that can clog the screen.