1989 Buick Skylark Have to use hammer to start

When I turn my key over, nothing happens, no clicking, no nothing. I have taken my battery and starter out and have had them tested at O'Reilley's and Car Quest and they say they are all right. I replaced my ignition switch since it is 20 year old original equipment. The only other thing I could think of is a problem with wiring or the neutral safety switch but then I thought I'd try something. I tapped on the starter with a hammer and the car started. What's going on here? A worn out bendix or improper (or lack of) shimming on the starter? Thanks!