Cinestill DF96 Developer and Fix Monobath for Black and White Film (1L)

I've developed three batches with this developer. Each time I developed a roll of 135 & 120 simultaneously. Here's my results by batch: 1) Everything came out great, was super excited for this 4 minute and done developing. 2) Both rolls were underexposed.(Possibly my fault as I did not add extra time since the instructions didn't mention this.) The 135 reticulated, but not the 120, which is weird because I thought this was caused by extreme temp change, so figured the issue would appear on both rolls, which it did not. 3) Both rolls EXTREMELY underdeveloped. (I decided extra time must be needed, so I added the exact time laid out by the CineStill website for this chemical) Was able to save a few shots by doing some artsy editing, but most are too dark to use. I've read this is a difficult chemical to make work and that everything must be exact, and I definitely don't consider myself a pro, but I've never had an issue with d76 or c-41. I use a sous vide so temp regulation is not an issue. And the third batch I added the recommended time to compensate the spent chems. I was really excited about this, but I don't think I'm going to experiment again. d76 is going back in my kit from here on out.