What could be reason if augmentin (amoxicillin and clavulanate) doesn't work on tooth infection? have dentist appointment in couple of days, what to expect in appointment?

Many reasons: Maybe medicine is underdosed, maybe not reaching the infection and need to do something at the local level. The dentist might be able to incise and drain if there is an abscess.

Resistant bacteria: Resistant bacteria is one possibility. The abscess may need to be drained to promote healing. Expect that the dentist will examine you, determine the cause of your problem, recommend appropriate care, explain your options, and potentially begin treatment.