I had a tooth abscess , then root canal, but the abscess has not completely gone.

Failure?: that means ether abscess was not treated properly before the root canal treatment,or the root canal treatment was not successful,and led to recurrent infection . Proper examination needed to determine the condition and treat it weather by re treatment for the RCT,or extracting the tooth and replacement options discussed. Good luck ;)

Time Frame ?: How long ago was the root canal finished? Is this the same infection that necessitated the root canal? Either way, see your dentist to have the tooth re-evaluated.

See your dentist: He/she must be certain that the infection is fully eliminated before the tooth is restored with final restoration. If the infection remains it increases the risk of post treatment pain and root canal failure. See your dentist for re-evaluation.

Reevaluate: Return to the Dentist that did the root canal. May have untreated small canals, residual infection, fracture. Evaluation by a root canal specialist, an Endodontist, is in order.