Bad starter, good solenoid. Good starter, bad solenoid...help!?

So here's my situation. I have a 92 Golf 1.6 turbodiesel, manual transmission. The starter that was on the car initially was working when I bought the car 1.5 months ago, but recently stopped working. I took the starter off and found it's really stiff, hard to turn by hand, etc. The solenoid is good, or at least was good while this starter was working. I have a spare starter which turns much more freely by hand, however the solenoid won't kick the bendix drive out to mesh with the flywheel. No junkyards had a diesel starter, so I went and ordered a new solenoid which will be here in 2 days. My question is, how would the bad solenoid be, well, bad? What would cause it not to kick the bendix drive out to mesh with the flywheel? I'd feel pretty dumb if it were something simple...