motor wont turn over?

I tried to take a few plugs out to, battery is full and have a charger/starter on it to help. I thought the same thing, maybe voltage was to low. But even with all the plugs out but one, when it get to that cylinder when spinnig over, it just stops there, like the pressure is too much and the exhaust is trapped, but like I said, I have the valve covers off and I can see that the intake/exhaust valves are opening and closing