1987 Mitsubishi Montero engine won't crank

This has me stumped. My engine won't crank. My battery is good, I just replaced the starter (although, as it turns out, the old one wasn't bad), and my engine will turn by hand (with a breaker bar on the bolt in front). There's a hard click when I hit the key, but it won't crank. I use the vehicle to plow snow, and it sits in a garage most of the year. It was working fine the last time I used it a couple weeks ago, and I've never had this problem before. What else can it be?

Thanks for any advice I get. It's greatly appreciated.