Look, less subtle this time : Curse has nothing to do with this. Modpacks on the platform are created by people like me and you. It would be better if you told us what modpack it is. Also, it doesnt mean the client and server have the same mods that it should sync. Modders can do sided calls and ignore another. Curse doesn't guarantee anything to work on its platform, it's a community one. As I said, go look at the Modpack, look for its issue tracker and either try to find a solution or submit a ticket there.