I would say that you are not doing the standard pin.
That is pinning via the arm alone and that leaves him option to escape by the hips.
I dont know how standard my pin is, but I definitely dont only use the arm. I am using the arm, my weight, and my legs.
I pin the arm using a spiral movement away from uke's shoulder. (some people use a circular movement which i believe is less effective). At the end of the pin uke's hand is usually bent past the shoulder.
At the same time I am using the leg that is next to uke to prevent him from rolling. (because I am yoshinakan, my inside knee is up). The leg is helping the arm pin and keeping uke's inside shoulder pinned to the mat. Since most of my weight is on the arm and shoulder its hard to roll out.
The pin is really quite painful, and uke usually doesnt want to try rolling out.
If uke is stupidly flexible (i hate flexible people... its jealousy), the shoulder doesnt really pin, the leg doesnt help enough and uke can usually struggle out. But that is where atemi come is