One other thing to consider: I was listening to Art Pennom's "Tank on Fire" podcast today --an episode on bucephelandra-- and he mentioned that, like crypts, buces can lose their leaves when changing from emersed to submerged growth or adapting to changed water conditions, such as after a large water change wherein the qualities of the new water aren't closely matched to the old. Assuming for the moment he's right, I wonder if you're seeing that here?
Oh, and regarding fertilizers, I gave up on the Seachem line: I've had much better results using an all-in-one, in this case Aquarium Co-Op's "Easy Green" and "Easy Iron."