Minecraft Forge mod(s) issue?

Summing up my issue: I run a private Forge server, installed mods, the modded items appear in the creative inventory when searched BUT whenever I attempt to place down any modded item they immediately disappear. So, how exactly do I prevent it from disappearing? The server runs fine, the mods appear as installed, they just won't let me place them.

In my experience that happens whenever the server has a different version of the mod or doesn't have it at all