After opening Avraham's wells, why did Yitzchak need to dig new wells?
Malbim: Yitzchak had more flocks than Avraham, and needed more water. Avraham's wells were of rain water (not flowing water), and did not suffice.
Ha'amek Davar: Yitzchak re-dug only the wells that they stopped up (26:18). The ones that they filled with earth were not recognizable; therefore, he needed new wells in place of them.