Wondering why you're being asked to spend coins on episodes of a Wait-to-Unlock story rather than the episode unlocking automatically? If you see the notice below, that means you've reached the most recent episodes!
 
 
If you click on the "?" above "Continue Reading" on the story's cover, you'll see that it has certain rules about which episodes can only be unlocked with coins.
 
 
A certain number of the most recently published episodes for Wait-to-Unlock stories will always be locked and are only available for purchase with coins. You can't unlock them using story coupons or by waiting.

If the story is still updating, new updates will make older episodes eligible to be read for free. You'll just need to wait until the episode you want to read has the hourglass symbol next to it, rather than the lock!
 
You can learn more about story pricing models here.