How Guns will be unloadable:

-Drop or find a Gun on/to the Ground
-Open your Inventory near the gun of the Ground
-Navigate to the Gun's Magazine and take it with Pressing "A" Button to your Hands.

(Now the Weapon have no Magazine but the Icon of the Weapon have still a Magazine, normally the should not be show a Magazine without a Magazine)

-Now the Gun are no more reloadable.

How to fix this Problem temporary:

-The Weapon needs to be on the Ground.
-select an magazine in your Inventory which are compatible with the Gun.
-Micromanage the Magazine from your inventory to the Attachment slots of the Weapon on the Ground.
(Hold "A" and Navigate with "LB, RB, LT, RT")

-now the Gun should have the Magazine in, and are again reloadable

I hope the Development can fix this Problem soon,

It can be annoying not to know how to fix the problem, it is better to fix the problem.

The core problem is the icon, if the icon indicates a magazine but no magazine is in the weapon, the icon blocks the reloading.

The icon does not change if you take the magazine of the weapon from the ground into the hands.