What could cause a persistent skin boil?

Colonization. You may be a staph carrier, and continue to reinfect yourself. Consider having a nasal swab done at your doctors office to determine if this is true. If so there are strategies to minimize the recurrence of boils.
See below. A boil is a bacterial infection of the skin. A persistent boil would result from infection with an aggressive bacteria that the body's normal defense mechanisms cannot overcome.