Re: Marcgravia flowering
Most viv specimens of Marcgravia are perpetual juveniles - the substrate-clasping foliage, which makes it such a viv favorite, is absent from the adult branches from which flowers arise. Those branches are arching/pending and 2 or more feet long.
The flowers are indeed amazing - their odd shape and organization are adapted for bat pollination. You won't see flowers unless you can provide for multiple, large adult branches which requires conservatory-scale accommodations. How large are the plants you're working with?
When I hiked in the rain forest in Puerto Rico, I saw alot of Marcgravia in juvenile and adult form. Out of 100's of plants showing adult foliage, I only saw a few in flower. I tend to think rtbaum is correct that only large adult plants produce blooms. The flowers in their pendant umbel are quite fleshy and robust and would seem to require an enormous investment of resources.