I’ve had that same issue with my D line plugging up. It would only happen if there was a long period (about a month or so) of no use. I am trying to remedy that by priming the line with RODI water after the line is used.Just going to share a word of warning to don't leave it too long to not run a prime/calibrate on Pump D the alk addition head.
I hadn't done one for ages & my 2 part doser had an issue.
Where the alkatronic was set to pick up & dose once it got below the action mode level it thought it was dosing but the line was crusted up so nothing was going in.
Had Fusion2 plumbed to it - think i might swap to something weaker to try help stop that repeating.
As well as slap myself to run/check that line a bit more often.