If a companion drops a weapon because it's been 'shot out of their hands', it means the weapon is in complete disrepair. Therefore, they will not pick it up again because as far as they're concerned, it is broken & unusable.
If you want it back you have to look on the ground and find/pick it up yourself, and take it to a merchant to have it repaired. (you can't self-repair once a weapon is completely broken, far as I know anyway, tho you can use a broken one to repair a not-broken one...).