Unfortunately it is a common problem. With age the plastic becomes brittle and do not hold the little metal runners in place anymore therefore causing them to stop working. One has to open the Beomaster up and glue the metal runners back in place. Its not difficult really just annoying. There aren't any replacement sliders so repairing the originals is the only option.
Dillen on this forum is very clued up on this repair, otherwise do a search on the forum on slider repairs.
Ben