duck problems

hi,  i have a silver appleyard that has started over the last couple of days found walking at times quite hard, an old friend of mine who has kept ducks for some years has checked the ducks legs and hips and can find nothing wrong with its legs at all, one minit he has to use his wings to help him get around the next hes walking around fine, hes eating well too, and doesnt seem to be in distress. im at a loss does any one have any ideas as to what may be wrong.