Check if there's a tiny switch on one side of your USB stick,and make sure that it's in the unlock position if there's such a switch. If there's not,you could try tweaking your registry.
Back the Registry up by selecting Export from the File menu before doing anything else. Then navigate to this key:-
where you should find Write Protect in the right hand pane. If it isn't there, right-click that pane and select choose New DWord, type Write Protect and press Enter. If you create it yourself, give it a value of 0 (zero) but if it was already there, make sure that it has this value-0.
Leave the Registry from the File>Exit menu, remove the stick and restart the computer. Try the stick again and it might have worked.