Zanussi ZWF14080G replacement belt alignment issue

Hello there, I recently moved house and the belt fell off my machine during transit – yep, some bad practice in there that may be the source of my current problem – the belt was not in good condition, it had a shape of the small pulley impressed in it, with the belt also a bit ‘crunchy’ – when I tried to put the belt back on I tore it. I managed to source a replacement and have been able to ease it into place around the two pulleys (it’s tight, but it should be!)… HOWEVER despite my best efforts as soon as I turn the pulley to test, or run the machine, the belt slips off. What appears to be the issue is that the two pulleys are not quite aligned. I can’t spot what the cause of this is, both the main pulley and the motor spindle appear to be in good shape, turning nicely, firmly in place, both secure in a rigid relation to each other… except that the belt keeps coming off. Any thoughts regarding this? I don’t feel it could be a major mechanical fault as everything else seems to be in order – it’s hard to see how the pulley on the back of the drum could be at fault as it seems not to be wobbly or at anything other than the right distance from the drum – the motor seems comfortably seated… is there a way in which the motor spindle could become recessed by a centimetre? Or perhaps there’s a small component that might have fallen off during the transit? As you can tell, I’m a complete amateur at this sort of thing, I felt that ‘putting on a new belt’ might be within my grasp but clearly not! Any help or advice much appreciated, thanks, Matthew