Hotpoint Aquarius WMA58 Belt slipping?

My hotpoint started to make a very loud noise when spinning when I next used the machine it went through a washing cycle but the drum didn't turn. Took the back off machine and the belt had come lose, this was put back on (not an easy job) and thought all was well but when used the machine again the same thing happen the machine went back to the original fault of making a loud noise when spinning had to stop machine it was shaking all over the place. Anyway the belt had come lose again. Has anyone got any idea why this keeps happening? Could there be a problem with the drum, the belt looked fine when I put it back on could this be the problem? Any ideas please....
Jane