zanussi zwt71201wa washer dryer

i have been out to two of these integrated washer dryers. model ZWT71201WA. PROD NO 91460603800. when fitting a new drum pulley because the original was worn. after fitting the new one the belt keeps coming off of the pulley at the drum side. I have now been out to a third machine which looks like it may be the same problem. it looks as if the drum shaft has moved forward slightly through the bearings which is making the belt run offline. has anyone come across this before or is it becoming a common problem?

thanks mark