Right shoulder losses strength

[bryanbankester]

Whenever I do certain exercises my right shoulder loses strength. Exercises such as:

When holding a 10lb dumbell in each hand and slowly alternating punches from waist to shoulder level in front of me. When holding a 15lb dumbell in each hand and alternating raising left and right in front of me to shoulder level.

There is no pain but my right shoulder weakens and even falls sometimes during those exercises. Never when doing heavy overhead military presses though. Only when doing those certain shoulder motions with hardly any weight. My left arm never weakens or falls and can handle lots more weight. What are typical causes for this?

[mjf100215]

When holding a 10lb dumbell in each hand and slowly alternating punches from waist to shoulder level in front of me.

Satisfy my curiosity... why exactly are you doing this?