5 yr old, uncleaned lines are my thought. I got good flow on 3/16 BUT I change every tee and connector in the system every year, plus I flush the lines after every season. Then last year I filled the entire system with calcium hypochlorite and drained it mid fall. Even then every tee and connector was changed. The I.D. of the fittings on 3/16 is so small that it takes almost nothing to plug it.
In fact even 1 yr old lines would often plug if not cleaned and new fittings. By the way, you should change the fittings after 1 day sap flow, in case anything plugs.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.