The Hornby turntable is really an oldie designed by Triang and is cheap (fairly) but can develop problems. It operates on a rotating cam with a peg which slots into a series of semi circular slots in the underside of the turntable platform. check that the turntable platform is close and even with the base and that the base is properly flat otherwise they tend to disengage. If you can find a Fleischmann HO turntable they are usually far better but far more expensive!