Same thing happened to me when i switched from cfl to t5HO. Had the t5's down to about 3" from the tops and was getting no stretch. I recognized that they were not stretching, so raised the light up to about 6", bumped up the veg nutes a little and they took of stretching. Not quite the way I wanted things to go, I was trying to grow up into a scrog screen but turned out more like "structured LST".