Gloria, I have the same issue when putting on my armor over the vest. I also use 1/4" elastic around the sleeves to keep the bells in place. I believe the issue might be that the elastic is pulling on the sleeves causing the sleeve to bunch up/move.
My fix is whenever I'm suiting up, I or I have someone help pull the sleeve out from under the shoulders and make sure the elastic is sitting properly around my bicep.
I like to put my arms out at the side so they can pull the sleeve out from under the elastic and then adjust/position the strap so it's going around my bicep and doesn't interfere with the sleeves.
Not properly fixed:
Fixed:
I hope this helps and makes sense!