It’s pretty common for the big strong Traps, Lats and Biceps to overpower the smaller muscles that support the “back” and “bottom” of your shoulder.
When this happens, it’s common muscles in the side of your neck become chronically tight.
Disks pinching and bulging often cause nerve problems. When that happens, it becomes difficult to control muscles correctly.
It’s not surprising you’re having trouble with the muscles of your upper back, and how they work with/against your lat. It may not be that the muscles are “weak,” just that your posture or shoulder position are slightly “off.” Maybe the nerve issue put them to sleep. Maybe your strong lat, trap and bicep are pulling your shoulder into a position where the upper back and muscles that support your shoulder are stretched out and not working properly.
Now, if you’re still following; Your tricep attaches to your shoulder. If your shoulder isn’t in the correct spot, your Tricep can’t engage and lock out your arm properly. The arm just won’t straighten out.
You’ve got to work on the muscles that Depress and Externally Rotate your shoulder and arm. If you can activate your serratus to get your shoulder “tight” and turn your thumb “up” to externally rotate your arm, you should be able to get things working.
This will probably help your posture if you work at it. And even out your high left shoulder. Lots more in here