I am a complete noob with such things so take this as you will. I have read a few times that BK doesn't stretch much if at all during flower, could that have anything to do with what is going on with it staying short? I know various plants (all plants in general) also take different times to bounce back and may not. Another idea is if you have the weed to spare or don't mind a seeded batch once in a awhile let this grow out and IF the BK takes change the sex and pollinate the tent =] That may be more work than you want though as I know if I did that I would want to wash the tent as to do away with the pollen.