Ingoing ph is a tad low, but how are you testing the runoff? If you're using those color-coded ph drops, the results are skewed by the tinting of the runoff water, making them basically useless for runoff.

How long have they been in the pre-ferted soil? MG 3 month formula lasts about 6 weeks before you need to start adding half-strength nutrients, but I have no clue how long the nutrients in the Plagron lasts.

How long have they been in the same pot? When you transplant, (if still using the Plagron) this will somewhat replenish ph buffering and nutrients.