A problem I used to always run into when I got to the mount and wanted to switch to an arm bar was that my opponents would keep their elbows really tight to their hips making it difficult to successfully transition to the attack. A good way to ensure that you can keep the elbow near the line of his shoulders and in a position to facilitate a transition to an arm bar is to never relinquish control under his arm from the pass all the way to the armbar. This way you can go directly from an under hook pass into a mounted Armbar never giving him an opportunity to retract his elbow defensively.
