Too many variables, could be the pump diaphragm, a tiny leak some where, a bad gauge or several other things. because the sap is moving slower I'd change the diaphragm first, it's easy. In fact, while I only use a diaphragm pump to push fresh water thru my tankless water heater (or for cold water too) I keep a spare diaphragm in stock as well as a spare pump. Just use this saying, "2 is 1 and 1 is none"!
Good luck.
Dave Klish, I recently bought 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.