Modpack Blocks Immediately Disappearing :(

[lokee]

I've setup a Hexxit server and most of the modpacks seem to be working, haven't run into any crashes yet... but my problem is that when I try to place a non-vanilla block, it instantly disappears! I don't have this problem in single player, so it has to be the way the server is configured. Could anyone help me out with this?

Edit: Also the Damage Indicator modpack doesn't seem to be working.. When I attack any mob their healthbar doesn't change, nor does it show the hit points.