Occasionally we need to do it twice
The most common cause of a need for root canal retreatment is contamination of the treated root canal system, i.e through a leaking filling, through recessed areas of gum tissue exposing more porous areas of dentin, or through decayed tooth structure.
Alternately there may be some missed anatomy which can be cleaned when the root canal is redone. Sometimes root canal anatomy is complex with very small canals that are difficult to find without special expertise and an eye for microscopic details. Or there may be bacteria in the root canal system which are resistant to conventional disinfectants. These problems are all best addressed by non-surgical retreatment: removal of the root canal filling material, reinstrumentation, disinfection and finally refilling and sealing of the retreated root canal system. This is usually done in 2 appointments to be sure the system is disinfected. Success rates are lower for retreatment than for the initial root canal treatment, but are still high, at least 80% depending on the circumstances.