There are a few possible reasons why your MW3 aftermarket parts aren’t unlocking. Here are the most common ones:
You haven’t met the unlock requirements. Each aftermarket part has specific requirements that you need to meet in order to unlock it. These requirements can include getting a certain number of kills with the weapon, completing a certain number of challenges, or reaching a certain level with the weapon. You can check the requirements for each part in the Armory.
There’s a bug. There have been a few reports of bugs that prevent aftermarket parts from unlocking. If you’re sure you’ve met the requirements for a part and it still won’t unlock, it’s possible that you’ve encountered a bug. You can try restarting your game or console, or you can contact Activision support for help.
You’re playing a modded version of the game. If you’re playing a modded version of MW3, it’s possible that the mods are interfering with the game’s unlock system. Try playing the unmodded version of the game to see if that fixes the problem.
If you’ve tried all of these things and your aftermarket parts still aren’t unlocking, you can try contacting Activision support for help. They may be able to troubleshoot the problem with you or fix the issue on their end.
Here are some additional tips:
I hope this helps!