We have probably all asked that question about certain gifted players who appear to be stuck in the rankings, usually because of inconsistency. And inconsistency can be caused by several things, but I think it is usually caused by a lack of mental strength, chronic injury, or--in some cases--both, because having injuries causes players to lose confidence.
The players on my own list include Gisela Dulko, Michaella Krajicek and Marta Domachowska. Other players who probably pop up on such lists are Carla Suarez Navarro and Kirsten Flipkens.
Who is on your list?