I had exactly the same problem. I've spent the last two hours checking for air leaks, then it struck me. "What if the water isn't getting to the pump because of an obstruction?"

I opened the pot and felt for the passage between the pot and the impeller (with the pump off and unplugged!) sure enough it was clogged ith vegetation! It must have found its way past the basket. Cleaned it out and the jet is noe like Old Faithful!