In some minecraftprojects/custom maps, it won't let you cheat because when you type /gamemode 1 or smiliar it will say you do not have permission to use this command. All you have to do is press Esc and click Open To Lan. It will say LAN World, Settings For Other Players. then it says gamemode, and then Allow Cheat, you want to turn it on (by click it). And then click Start LAN World. Now your allowed to cheat!! but when you pause the game, it wont pause and all you have to do is save and quit and join back in and everything is back to what it use to be. Have Fun!