If you hover over the weapon and you can see the augment, then it is client side, indeed. Else, it is server side, most definitely.
Fact of the matter is, there is a whole chain of packets that are responsible for handling the new Augmentation system. The easiest way to keep track of them is by activating the NETCHAT packet debugger from ALT+G. It is not a debugger per-se, but it would help you see the packet exchange between client and server, which should then show you where the issue lies, namely the client->server packet, whose trail you should follow until you find your issue.
If the other weapons are working, then it is quite possible that you have missed adding the new custom weapons somewhere in the datapack (in some xml or wherever your server pack keeps such data).