Promix tends to drop in pH over time so you need to re-pot at least every 6 weeks, into fresh mix, and monitor your runoff pH in between. If you see it suddenly drop before then, you must flush with tap water set to 6.8-7 until it starts coming out at around 6.7. That's the high end of the correct pH for soilless, and you set it on the high end because it ALWAYS drops (unless your tap water is outrageously high, like 8, lol). IF it drops early, you need to look at WHY. Is your fert solution causing it? Is a salt buildup in the soil causing it? Is your water very 'soft' or loaded with organic acids, and do you need to amend your tap water with calmag to help buffer this?
Read the 'why I should re-pot' thread in the FAQ section for some info.