Recurrent boils can occur in many conditions, some of which are:
1. Diabetes mellitus (which you probably do not suffer from)
2. Other endocrinological disorders
3. Liver disorders
4. Immuno-deficiency
5. Reinfection from a reservoir of infection
6. etc.
So, if your GTT is normal and still you continue to get boils, further analysis is required. At the same time, it should be kept in mind that there are occasional cases where no cause can be found.