What Causes Cracking Of Skin In Between Buttocks?

The crack along my buttocks keeps cracking open. It will start to heal and the in a couple of days it will crack at the top or the bottom. Have tried using A&D ointment and diaper rash creams. Nothing seem to heal it to the point it no longer cracks. When it opens I have slight bleeding and it is sore to wipe with a wash cloth.