Starting issues

When trying to start my car, the ignition turns, I hear a click, and nothing happens. Lights are all on in the car, battery has tested good. Alternator has been replaced in the last nine months. Had it towed to a shop, they replaced the starter. Went to pick it up, and had the same problem in the parking lot, would not start. Thought the starter was bad, so they put in a new one. Picked it up, drove it to and from work for four days. Went out to drive it on the fifth day, would not start. Same issue. Towed it back to the shop, they replaced the starter again (bought a different brand from a different place) and brought the car home again. Drove it to work today, no issues in the morning. This afternoon after work, will not start again. I have a hard time believing it could be another bad starter, but not sure what else to check. The shop is not quite sure either. Any ideas what to check next?