Sounds as though one, or some of the teeth on the engagement cog on your starter are getting to the point where they are not turning the flywheel.In that case you need to replace the cog (sometimes it's called the pinion) on the starter and all should be well again. Another reason (more unlikely, but not impossible) may be that if its a pre-engaged starter the pinion can get sticky on the armature shaft, a good general maintenance cleaner spray will sort that out.Work the cog up and down the shaft a few times to free it off. Best thing is, take the starter off and have a look. Clean up the solenoid too as sometimes they do strange things. Don't forget to disconnect your battery before you start work.
Hope this sorts it out. You can bet it will let you down if you ignore it!!