There are 2 possible issues I can think of off the top of my head..
1) Mechanical deficency
Your ankle to knee, force vector, may not be linear enough by the time it's side to extend your leg. It should be pointing to your front hip. If it's too high or low, you're not going to get good extension.
2) Strength/Muscle control
To drive your back leg through, you need a decent amount of strength to get into the position to do that, and motor control to drive the leg.
Power cleans should help this, you'll gain explosive strength and recruit more muscle fibers, thus increasing motor control.
If you have taken some time off in the gym, this could very well be the reason, even if you didn't have the problem earlier.