I have had bad pumps do exactly what you describe. Borrow pump from neighbor or friend that they use regularly. If it works you got a bad pump. If not its probably the valve stem. You can usually by a package of shrader stems and removal/install tool pretty cheap. They just unscrew/screw in. I did have bad pumps ruin a couple of stems. I also use my tool to remove good stems from any damaged tubes I through out.