Tracked robots rely on skidding to turn, so it sounds like what you are describing is probably normal. If you think the skidding is excessive, you might try cleaning the tracks.
If the skidding is causing some inconsistency in the turns, you might consider implementing a similar turning logic as the maze solver example program for the Zumo shield. Instead of turning for a set amount of time, that example turns until the reflectance sensors detect the line again.
Brandon