Urgent dental advice, i'm getting an abscess on front tooth, already had root canal, how can this be?

No source control: Recurrent infections are at times, caused by inadequate removal of the source of infection. I'd be worried about an infected root canal, infected bone (osteomyelitis), and infected tissue that's too far gone for antibiotics to work. The abscess needs to be drained otherwise, complications like meningitis can occur.

Endodontic Failure: A tooth that has already had a root canal can require additional treatment if the root canal fails. This is not uncommon and the tooth will likely need a second root canal or possibly additional surgical procedures. See you general dentist for evaluation.

Failing root canal: Root canals can fail if there is an inadequate seal or insufficient disinfection of the canal system. Fractures of the root may also cause an abscess. Sometimes periodontal abscesses may be associated with root canal teeth even though the root canal is fine.