My Bta always hides he never come out.
I've had him for 6 months now. He's always under the same rock.
All my parameters are in check. Tank is about 2 years old
How small is it? BTAs have a tendency to hide when they are very small. As mentioned, it could also be too much or too little lighting. But bottom line is that it's gonna go where it feels comfortable, not so much where you want it to stay. My BTA never came out when it was small, but then gradually moved to where it felt comfortable.