I had the same issue on mine over the winter, changed the battery, solenoid and starter switch and cleaned every connection. Even put a better earth lead on but none of the above made mine fire up. Hit the starter and all I got was just a single click from the solenoid and nothing else. Mine turned out to be the bushes in the starter motor had worn out so weren't making the connection as they wear over time. So order some new bushes from Ducati John in Leeds. The starter motor is a bit of a ball ache to get to but easy enough to replace the bushes once the starter is off the bike. Or just replace the starter motor unit altogether. Starts first time every time no issues.