The starter (or other things) can break a tooth off the ring gear on the flexplate, and then if the starter is reengaged at or near that broken tooth there will be nothing for the starter to grab on to. It's not super-common, but it can happen, especially if something (e.g. a broken part) gets inside the bellhousing.
Even though your issues could be related to this, it's not the most likely of causes. It's far more likely the solenoid in the starter that pushes the spur gear out (to engage the ring gear on the flex plate) is bad, or the clutch. More likely that it's the starter, in other words, so check there first. (But while you have it off, you can see in to the ring gear in the bellhousing fairly well, so might as well inspect that too.)