If you tend to use the same exercises all of the time, then there is a good chance that you have developed some structural imbalances. Structural imbalance is a generic term that describes a large variety of imbalances. These imbalances can occur between the muscles of the front and back side of a particular limb or between the same muscles of alternate limbs. The best way to prevent this is change your exercises every 4 to 6 weeks and be sure to regularly include single leg and single arm lifts.
Once an imbalance is established, there are a few great exercises for correcting them. For an imbalance of the legs, be it between legs or between the hip flexors and extensors, the split squat is superb.
Try switching to spilt squats for 3 sets of 10 reps for your next 6 leg workouts. This is also a great on the fly substitution for squats or deadlifts if your back is still tired from the last workout.