My CS-GO skins are not working in offline or in online multiplayer mode. I've seen this question, but it talks only about online multiplayer mode and my skins are neither loading in offline nor in online.
I also had this kind of problem and I found some solutions that might help you
You can turn off Windows firewall that might be blocking your data
You might not be able to access skins in offline mode as they are loaded from the internet in some conditions.
If you don't buy the gun yourself you won't get a skin. You must have the gun yourself and not by family sharing
Hope this might help you.