Starter solenoids usually wear out at the contacts inside. Sometimes you can smack it with a hammer to get them to crank once more, and if it works, you're really on borrowed time.
My '61 Apache pickup left me stranded every 1 in 100 times. The starter would click, but no cranking. If I got a jump off someone else, their running/charging voltage of 14.6V plus their extra amperage was enough make the connection inside the solenoid. The solenoid is not serviceable, but here is what the contacts looked like inside after I cut it open to show you:
A brand new starter was only $98 CDN.