Why does my daughter's eye turn inwards after squint surgery?

My 8-year-old daughter underwent strabismus surgery (esotropia) in both eyes 2 and a half years back. Things were absolutely fine. However, nowadays one of the eyes has started turning inward, when she focuses at closer objects or looks at me in the eye. This happens even when she was down with fever. Why does this happen? Will she need one more surgery? What are other treatment options besides surgery? At the time of surgery she had 20/20 vision in both eyes and the surgery was done in USA.