Hello; Bare bottom or gravel /sand will not make any difference as far as the colonization of the bb goes. There is some advantage to a bare bottom in that you will be able to clean up the detritus/ mulm (aka poo and such) much easier because you can see the stuff. ( one more tid-bit - you are the clean up crew. Catfish nor filters will do this part for you. In fact some catfish, plecos, just make it worse.)So a follow up question, I kept the tank bare bottom. Is this part of the issue, and if so will the tank still be able to creat BB with a gravel/sand bottom?
I stopped using sand (at least the finer sand) decades ago. I use substrate from bb to small pea size.
I also am still using UGF under my substrate and run them with air.