In the following class, Rickson Gracie shows little adjustments that will help you never lose an armbar from the closed guard again, even when your opponent is trying to protect themselves. 

The most crucial detail of this position is to avoid stepping on your training partner's hip to start moving, because you will find yourself in a weak position. 

So, what you want to do is lift and angle your hips and, at the same time,  use your legs to climb your opponent. You must ensure that one of your legs will go over your opponent's shoulder and the other right under their armpit, keeping the pressure with your knees. 

Watch Master Rickson's demonstration, and his students practicing, to understand the angle you should have and how to use your legs to trap your opponent's arm.

Keep your eyes and ears open for all the details. Keep training hard!