Like the other person said. They come out at night. My clown pleco is a lazy bum during the day some days she will get around but for the most part she just sits in her cave and keeps everyone away from her space. She does come out to get some algae wafers but is still shy about letting me watch her. My BN on the other hand was active and happy to moment I let her out of the bag. It depends on the fish.
Make sure you give her lots of places to hide so she will feel safe and not get stressed. Adding more hind outs can help. Also she is new to the tank. She needs to adjust to this new world. Give her time I am sure she will move and set up home in a cave she finds.
But keep an eye on her and the other fish watch for any illness because unless you quarantine before adding you don't know if maybe she does have something. Most likely she just needs to adjust.