I know it's crazy no one is mentioning this issue but I finally figured it out. Hopefully it's the same for you but for me it was if I had any mags or ammo in my vest or plate carrier it wouldn't let me combine the mags and ammo. So you have to put all your mags and ammo in either your shirt, pants or backpack if you want to load your mags without having to drop your ammo on the ground until they fix this issue.