There are a couple of reasons for this:
A: It’s a game engine bug that we can’t help. There is thankfully a workaround. If at any point in the weapon modding process, you have categories disappear, back out of the bench then equip your half-weapon & re-enter the bench. The missing categories should magically reappear. If they don’t, drop or scrap any others in your inventory and then try again.
B: You have an outdated patch installed that is affecting the weapon and its modifications. 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.