It could be a combination of things. I am going to suspect the 1/2" line size, first off. But, it could be that the fittings and valves you are using. If the fittings and valves have much smaller passages, they could be restricting the air flow at the fittings themselves. By the nature of flexible plumbing, fittings for specific line sizes have smaller passages...some more than others.

It's possible that the lines are getting water in them and that water is harder to push than air, or harder to push air around it. It's very hard to push water through a diffuser membrane too. Do your diffusers have check valves in them? Any leaks in the lines?

Once the system has been running a while and one diffuser dies back...it's possible that the pump is getting hot and dropping capacity. I'm not too familiar with diaphragm pumps, but it's something to look into. Do you have a pressure gage so you can see if the pressure changes as the air flow changes? The pressure should change as the air flow weakens so long as the pump is working properly and it may be exceeding the pumps capabilities.

The HB120 should be more than enough pump to run two Matala's at those shallow depths. I'm thinking line and fitting restrictions or it's getting water in the system. Aside from the problem you are having with the base system...I am wondering if 2 diffusers is adequate for your pond size, however. I say this because I am sure we can get the two diffuser system working better, but will it be enough to properly turn all the pond water over so that aeration benefits are achieved?

Welcome to PB!

Fish on!,
Noel