As someone that used to always get confused why I couldn't used my newly altered items when I would restore or add things to them, there is an easy solution to this problem. You select a different weapon in that slot and then reselect your new recently changed item. You can be alive, dead, spectating, in the main menu, or anywhere and this will work. Exactly like what Valen said but you don't have to carry it the whole round. Simply select, deselect.
The reason this happens in the first place is because when you alter an item (example: add a pro-KS kit to a detonator, changing it from a non-KS detonator to a pro-KS detonator), it basically removes the old one from your backpack and creates an entirely identical item plus/minus anything you did to it. So as human said, it tries equipping the old version, except it doesn't exist anymore. It still might show the new item in your slot, but the game is only reading the old one. Hence why selecting another item and reselecting the old one (putting two different items back in your loadout) basically refreshes your game and now recognizes and uses the new item correctly.
Hope this clarifies anyone's future troubles with this. Definitely not necro-bumping.
