promix= peat heavy
many folks love the stuff; i distrust it.

sounds like my initial diagnosis was dead on! lol
ph+wet
you shoudl get more drain holes going in those cups so that the soil almost dries out between waterings, and the peat can be trouble anyway... if you have a way to check your ph, do it...
keep pH between 6-5 anf 6-8. peat tests in the mid to high 4's... promix is meant to have lime in it already but i am like, meh, leyt me see that with my own eyes....