Well, the problem isn't regrowth cast itself. Even with only rejuvs in macro, if 1 cast fails then the whole macro breaks. It must have something to do with all castsequences advancing even though 1 of them fails, eventually causing 2 or more castsequences to be locked in same step or something like that...

I do occasionally use LB when more healing is needed, but i prefer to use regrowth+rejuv since they last longer and don't burn my mana too fast.