my whirlpool awm1866 won't drain?

it will wash and spin (when manually drained) but won't drain at all. the draining pipe isn't blocked and neither is the filter. the pump starts making noises at the right point but no water leaves the drum. the impeller i can reach at the back of the filter opening turns fine and doesn't seem blocked. is it a new pump needed? its quite an old machine (2nd hand) but has been reliable for several years although it has been recently moved.