Ok if this is what I think it is its due to lag. Although there are a few questions I have to ask to be sure. Has Hexxit just developed this problem or has it never worked? Here is what I think you should do. Go to options in the very start menu and go to video settings. adjust your settings. Make your graphics fast instead of fancy, turn render distance down also make your particles low. These all help reduce lag if its lag issues (I suffered from this sort of thing when I first got Hexxit and the other mod packs) otherwise reply to this post with the answer to my first question. Hope this helped.