Mods not working for other players

I have mods in my mods folder for my server and I also have mods installed on my client, but whenever I (or another player) attempt to place down a modded item it does not place. Also, when one player is holding a modded item other players cannot see it in their hand. We all have all the mods installed on our PCs. However, mods like the player models mod work and we can see each other's updated models.