Over time, it's possible for a tumor to develop resistance to treatment. This is when cancer cells figure out how to survive against treatments. This might happen when various treatments kill the cells they know how to kill but don't work against every last cancer cell. The cells that escaped the killing-effects of earlier treatment are called resistant cells. They survive the prior treatment attack and eventually grow. This is how recurrence can happen.
To get rid of these resistant cancer cells, you need new forms of treatment that work differently from treatments you had before. Sometimes a second round of treatments might be able to get rid of all the leftover cancer cells. But in other situations, additional rounds are needed.