Well after upgrading to 1.8.1 (bukkit ver 1149) some players cant place items, the block vanishes after .1 seconds and it does that with any block. it was working fine on 1.7.3 but since then a few users has had this problem.
Make sure that they aren't trying to place blocks near the spawn. Also, most of your problems with 1.8.1 will get fixed when a recommended build gets released. With that said, you are using a non-recommended build and need to accept the consequences that come with that.
