Possible lockout, what could've caused it, all 3 get the same water & feeding

Tough one, all your environmentals look on point, maybe spiking the rh a touch high at hight for flower but nothing that should be effecting growth in the way you have. Noticed you added nematodes and other predator bugs around week 2-3. This is not recommended mid grow as it can have unexpected results. That aside... Ph going in 6.5, 5 coming out. Means the plant medium is adsorbing all the stronger bonding + cation sites and replacing them with weaker bond Al3+ which is leeching readily to your runoff. In acidic soils (pH 5.0), phototoxic-aluminum (Al3+) rapidly inhibits root growth, and subsequently affects water and nutrient uptake in plants. Your ec is also incredibly high meaning you are feeding more than the plant is using and it's most likely to blame for the build up in ec leading to a drift in pH over time. At a pH level of 5, an excess of calcium and magnesium is unlikely to be a problem for plants because at such a low pH, the plant roots cannot effectively absorb these minerals, essentially causing a "lockout" where calcium and magnesium become unavailable even if present in high quantities; this is due to the acidic conditions hindering their uptake. Flush to reset pH, drop nutrient schedule to 50% and up it accordingly until she bounces back. Take ec after each watering , water frequently and only fertilizer when ec drops, add appropriate ppm to raise ec to desired level. Hold the line. Good