Which wormer did you use, Philben, and how often do you treat your flock? When were they all last done?
How old is the hen, and what breed? Is she a breed with particularly frilly knickers?
After you've cleaned her bum, how long before you can see more mess? (Does it just gradually catch on her feathers and accumulate, or does it appear as an attack of the runs every now and then?)
What sort of mess is it? (colour, texture) What are her other droppings like - (Since you say you clean her up every 2-3 weeks, she's probably not got bad diarrhoea all the time?)
Does she roost on a perch, or in the nestbox or on the floor?
Are you using Vaseline because her bum is sore, in which case has she any evidence of lice or mites, or even feather-pecking from the others?
What are you feeding her, and how is her appetite? Does she go out to grass or just stay in the run on mainly dry food? Does she seem to drink a lot?
Any other symptoms? Are the other birds OK?
Apologies for so many questions, it was just some of the possible reasons for her problem that came to mind - if you could give a more detailed description of her lifestyle and symptoms, probably someone could come up with some suggestions.