I have cycled daily for most of my adult life. Three years ago I injured my right shoulder. I don't recall any impact or anything strange. However my shoulder/arm became extremely weak. Two years later it did not improve. I ponded the problem extensively, but found no cause, hence no solution.
Last year I figured it out. I carried my bike up stairs by lifting it on my shoulder. I walked up and down steps at work daily for years. This year I stopped putting it on my shoulder and the shoulder healed. Power and flexibility has returned.
Plerase be careful carrying a bike on your dominant shoulder. It may cause injuries over time.