dogs that scoot their bums along the floor because of needing anal sacs emptying can corse infection doing so...so altho the dogs sacs have been emptyed your dog may well have abit of infection left their which can also still corse em to scoot along the floor.

i got that info here, have a read of this it may help. Other Causes of Hindquarter Conditions:

Also as jo-p said...depending wat wormer u used it mite not be strong enuff.