Just an idea, dunno if its too crowded in there. If you wanted to, you can get some string or yarn, tie it around your tallest branches ((like 10 inches from the top of the branch)) and bend them down easy, making them lay down/go sideways without snapping the branch, you don't want to do it too tight or use too much force/pressure, but just enough where the tops of the branches are pointing sideways and not straight up to the tents roof. kind of like low stress training but not as drastic. this way you may have more room to grow. - you usually do this in the veg stage when the plant is smaller. but it looks like you still have flexible branches and just starting flowering stage.
The stretch issue can be genetic, space between plants, high temps, weak lighting, or too much nitrogen, distance from the light during its grow cycle, lots of different reasons.