I would agree that there has got to be something that has changed but I can't imagine that it's clogging that quickly. I have a 9 inch diaphram diffuser in my pond and the pressure required to get air through it hasn't changed measureably in 4 years. I do not run my air 24/7 however and that could tend to keep mine clean by excercising the diagram every time it goes on and off. I also don't think it could have dug itself deep enough to add 3 psi, it would have to dig itself over 6 feet deeper. I'd check to see if perhaps an air line got kinked or something like that. My diffuser came with instructions on how to blow it out. You give it a blast of air at a higher PSI that will stretch the diaphram and open the pores so they clean out. You might try this to see if it causes the pressure to drop back down. Don't us too high a pressure or you'll blow the diaphram right out of the unit. Check with the manufacture for maximum psi rating.

Gotta get back to fishin!