When you say you've tried everything, are you referring to exercises or the method applied to performing these exercises? Furthermore, have you given power shrugs a go? Here we're delving more into the nervous system, awaking those nerve endings within your trapezius muscles like never before. This in itself would create a vastly different stimulus within your trapezius muscle fibers. How about principles such as ascending followed by descending? Have you tried increasing the capillaries within your trapezius muscles, creating enough capillary density within that area to ensure maximum nutrients delivery to the cells therein? You may be a victim of nutrients starvation there, due to a lack of optimal blood delivery, stemming from (but not exclusive to) a lack of capillary density within that area.
How to increase capillary density? Load the bar with a weight you're able to do (say) 50 reps with, and do 100. It's perfectly fine to take a 3 or so seconds of breath on your way to reaching 100.
So, high reps have the effect of increasing capillarisation in muscle tissue-(simply defined, capillaries are the tiny blood vessels where blood cells release their nutrients to the rest of the cells in the body). Delivering fresh blood to feed; nourish, and supply much needed oxygen, whilst at the same time flushing the area to get rid of metabolic waste products.
You could be suffering from stagnation of nutrients delivery and waste removal from the trapezius muscles. Give this injection of 100 reps a go, and see how you respond and feel afterward.
All the best Adrian.