Ah, that's probably it. Sometimes plant needs are counterintuitive! I suggest not fertilizing them in the spring, either. Residual fertilizer will affect the growth for quite awhile. Since the plants are young, light pruning only. If your goal is to get it more upright, perhaps remove the longest, floppiest stems all the way at the base and leave everything else. That's a tough plant, it doesn't require much care at all.