I have a server that me and my girlfriend play on. We went loking for dragon eggs but we didint find any, so i went in to creative to spawn two of them but when i place them the disappear as soon as i place them like when you try to spawn a zombie on peacful mode, (we play on easy mode.) I can see the eggs on my creative menu and we both have the mods installed both on the server and on our clients so i dont know what the issue is.

Make sure the mod is on the server as well as your clients.

Also make sure the version is the same between the server and the client and that the server is loading the mod.