Well I'm no soil pro but in googling Pro Mix it's called a peat based medium.....peat is certainly not an inert medium so I think you'd have to call it soil and treat it the same way.

7.1 is too high.....i'd wonder about overfert/fert buildup as being as issue as well as those conditions can raise your ph too. Aim for 6.2-6.8. Again....photos could make a huge difference.