Hi!
This post is quite old by now, so you probably already figured this out by now.
But just in case you haven't, your readout object for the stretched length of the hierarchy ("right_arm_stretch_end") is not parented at the right place in the hierarchy. So it doesn't move with the controller and therefore the math in your stretch system keeps on believing that it stays the same length all the time. Parent it under your "right_arm_ik_ctrl" and it will work the same way as the left hand.
However, your stretch system doesn't work all too well. Even on the left side where it is "working" you get a pretty big difference between where your control is and where the joint is if you stretch the length to 2xlength. This is largely due to the fact, that you are using scale to stretch your joints instead of orienting them correctly and using the translation of the main axis. Eventhough figuring stuff like this out on your own is fun and can teach you a lot. I would recommend, that you use a more common stretching solution for your rig. Also because using Joint scale will cause you problems when using blended skin weighting. which is pretty common to fix volume problem is the wrists.
I hope it helps!
