Do you know what kind of worms they are? Can you post a picture?
I wouldn't rely on DE to get rid of an active case of worms. The first thing I would do is bring another fecal sample to your vet to determine if these are the same type of worms.
Depending on the type of worm, you may have to treat more than once to break the life cycle. Large roundworms, for instance, can be killed by worming, but the adult worms have already shed eggs out through your chicken's poo, and those can be reingested by your chickens. Treating again a couple of weeks later breaks this cycle.
Good luck! It's an icky aspect to chicken keeping, but hopefully it will be over soon!