I have 3 in my tank,1 RBTA which recently split and 1 GBTA,the RBTA has never bubbled for me but the original that it split from has splits that are bubbling all the time,my GBTA had never bubbled until it was sick for a while,I nursed it back to health and it practically stays bubbled now,they're so much nicer looking when they're bubbled but it's hit and miss with that....I think it's multiple reasons that they do it but I don't know what those reasons are LOL
I don't think it has anything to do with water flow or lighting intensity as they will usually move until they are comfortable....water chemistry probable has something to do with it,water temperature maybe,lighting color may be part of it,type of food they receive may be part of it...who knows...it would be great if someone could figure it out though :D