In my experience, the times I have seen this has been a partial release of the tendon at the origin. It is my experience and that of of many of the top therapist's ( Kevin Wilk, DPT ) that at 6 months you should be much farther along even if there is no absolute model just " accepted "norms. From what I understand it is not a matter of having some end range weakness but absence of strength over 1/2 your range. This is not normal. I assume your surgeon has observed this. You can have a repeat MRI without harm
If this is the case, you will not be able to further strengthen. If not you should be ale to. You do not need stretches as you have full PROM, at least into ER. I would be curious to know what your scapula stabilized IR is. Look up sleeper stretch
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Please realize that when referring to strengthening I am only addressing ER.
We could always try ER isometric or eccentric. Five months of PT is more than I have ever spent except for WC heavy laborers so more of the same PT may not be the answer. I tell patients if they can and will do any exercises at home they should not do them in the clinic. This wastes your time. You should utilize the therapist for what you can not do