In my opinion the standard mounted arm bar, at lower levels, is an explosive move that must be done faster than your opponent can pull his elbow in. This is because at lower levels, people lack the technique to keep their body tight enough to trap their opponent's tricep, so execution relies on speed. As you get better in live rolling, you'll notice that you actually perform the move slower but tighter, which naturally develops into an S-mount control position before hitting the arm bar.
There's a reason why the mounted arm bar is a "basic" move and it's not because it's easy. It's basic because it contains multiple mechanics (timing, trapping the arm, pivoting off their chest, securing the arm bar position) that will take forever to master. De-constructing the move into multiple steps and analyzing it will yield key observations such as learning to control and isolate the shoulder during the transition, how to pin them during the rotation, and why a deep arm hook works better than a simple elbow trap (when fighting an opponent who knows how to defend properly)... etc.
So try slowing it down when you drill it and feel out each mechanic. Understand the S-Mount, because you are passing through it anyways even though you don't know it.