How much fresh air are they getting ?

Sorry but just a temp difference won't do that or how do they get buds to grow in Hawaii ? Temps are always 90 or so.

Sounds more like PH lock out or to me it sounds like 6 weeks is when they seems alittle slow but pick back up and put on the weight. If they don't get enough fresh air meaning CO2 thing slow down to a stop. 40 plants and 10- 1000 watt light really burn up the co2 in a room.