Chicken with a dirty butt

[McGoo]

I have a 1 year old hen who has had a dirty butt on and off since I got her at 3mos old. Recently I noticed it was real dirty - poop hanging from it... so I cleaned her up and she seems fine. Since there doesn't seem to be anything else wrong with her - I don't really know what this is. Any ideas?

comb rosy, laying regularly, I spread DE on her, the other hens, and coop every so often.

What should I be looking for?

Thanks all.