When unscrewing something the thread may be dry or even rusty. If you feel resistance add a drop of oil and screw it back in again; this will take the oil into the threads. Back out again and if you feel resistance again repeat the process. Keep up this in and out process without applying too much pressure. You may only get it a fraction of a turn more each time but eventually it will unscrew all the way. If you just keep unscrewing a dry thread and it seizes it will just tear or strip the thread off; especially very small or fine threads.