Possibly just getting used to the new surroundings. A lot of different fish can take some time to get used to new digs.
By the same token, it could be that they're completely nocturnal, not sure on upside down cats. When we first picked out fish, we got a synodontis that we would never see. You could see her in her favorite decoration, but she never came out, except at night, when we could not see her.
Like we said, not certain, but a possibility.