You could be maxing out on the cable, pinched cable, improperly adjusted rear derailed. As long as everything spins you don't need lubricant. Also wd-40 is the worst thing to put on a bike. Never use wd-40 unless your trying to free a rusted out door hinge so you can replace it. You'd be better off taking it to your local bike shop. Though if it's a Walmart bike they probably won't have parts and then you are sol.