Yesterday night I was AOG at an airport not far away from home where I went to get fuel. Master on, EIS showed battery at 12.6V, turn key to start. I hear a single click and no movement on the propeller voltage drop to 11.1v. An hour of hand propping assistance ensued but an o320 is hard to hand prop. Well I tried turning the starter again and the starter now works.. I guess movement of the engine from the attempted starts during handpropping fixed something. I fly home and now am wondering what is the problem. The starter is 4 years old, skytec 122-NL, I don't know how old the solenoid is maybe 12 years but my first thought was to replace that., What is the likely cause for the starter not working?