Enging will not crank

For a while my truck would do nothing when turning the key to start on the first try but would start on a second or third try. I thought I had a bad spot on the starter and so I replaced the starter and its fender mounted relay. The truck started right up so I thought the problem was fixed but the next time I went to start the truck, the same problem existed. I change the ignition switch below the steering column, but no change. There are no blown fuses and I can start the truck by jumping the starter relay. I am unable by myself to turn the key and check for voltage at the relay. Any suggestions? Thanks.