Agree, it is difficult at best to tell if a BTA will bubble or not. Tthe lack of bubbles is not a positive ID. Although, the presence of them is.
I got my GBTA from RCA that was beautifully bubbled when I got it but in about a day or two in my tank the bubbles deflated and have not come back. I see the semblance of one every now and then but that's it. When I had an RBTA that I got from Andrew it was the same situation.
I've read in other forums that a working theory is that in low light situations they bubble to increase the surface area for the zooxanthellae which makes sense.