i hadf one and that was the same sometimes it was days untill i saw it again they like to bury themselves in sand and hide alot they aren't very active at all so just be patiant and stare at the tank lol, they get around 6"
I love my banjo catfish!! Mine is like yours and only comes out at night after about 30mins of darkness. Turn all your lights off and wait. I usually throw in some food late on for it and he does go dashing round for it.
Its always good fun when your syphoning the gravel and are trying to find the catfish at the same time.