There are a couple reasons for this:

A: The weapon somehow spawned without a Caliber Conversion Kit (CCK) applied. Go to a Weapon Workbench and try changing the Caliber. You can use the vending machine and Pre-War Money to buy parts if you need to. If a CCK slot is not appearing, be sure to equip the affected weapon, or remove any others from your inventory, and then check the workbench again.

B: It is most likely being caused by an outdated or incompatible patch. Find and remove the troublesome patch.

If it is still not working after trying the troubleshooting steps mentioned above, then do let us know via our Discord and we’ll take a look.