I'm a bit late to the party on this one, but having done a little reading, as I currently have the same problem I think the issue may be the Bendix clutch.
It's likely that the solenoid nis correctly engaging the starter, but as the Bendix clutch is worn / failing the shaft is spinning freely rather than driving the Bendix.
Mine was starting first time every second turn of the key, but it's now every third!