Forum:Crafting items seem to disappear

so what seems to be happening is that i run a server me and 2 friends go on almost everyday well at the start everything was working smoothly but now i have hit a major snag. when ever we try to craft a tekkit item it shows up in the crafting result for a split second then disappears so far this has happened twice starting a new world after the last time any idea on how to fix this? please?