The this happens to any muscle group is because you have been doing the same exercise for too long and hit the comfort level.
In ordr to be stiff and see more gains, you have to breakdown the muscle again and the best way to do this is to incorporate what is called a "Static hold" in to the rep.
Sounds complicated but it quite easy. All you have to do is stop and hold half way through the rep for 3-4 seconds going up and then 3-4 seconds going back down.
Try it and you'll notice a big difference.
PS- These should only be done once in a while. You can also try increasing the weight so that instead of being able to do 6 reps, you're struggeling bigtime to hit 4.
Cheers, Elimidate