Problem with skimmer: no air bubbles

I have a 75 gallon salt water tank with a Berlin protein skimmer
attached to the wet dry . The tank has been up and running for over 2
years now, and the skimmer worked great for a while.

Recently the pump "died" so I installed a replacement pump and it
worked okay for a month or so, but now there are no air bubbles inside
the skimmer. Water is flowing through the skimmer just fine, but
there are no air bubbles at all.

I tried removing the pump and cleaning it. I've also tried adjusting
the air flow and that doesn't seem to help.

Has anyone seen this problem?

Thanks,
Mark