Is that the starter motor solenoid needs 12VDC. If you're batteries a little down, and/or you have some minor voltage loss, then it won't turn-over. You can try to clean things up (as Larry and others have suggested) -- or you can put a relay with a wire connected directly to the battery to kick the solenoid. That should solve the problem if that's it.

You can jump the solenoid and see if it starts --- if that's the case then it's not the starter. If that doesn't solve the problem, then it could be the starter motor has a problem and may need to be rebuilt.

Usually I think it's the first, rather than the second issue.