Solution: Go into your Spam/Junk/Filtered folders, find a Sheldon e-mail and label it as "Not Spam", and that should take care of it. The problem is on GMail's end, unfortunately.
This happens from time-to-time on the bigger mail systems like Yahoo, GMail, or in corporate/government systems: Because their spam algorithms are shared across users, when something flags the Sheldon deliveries as "spam" in their system, then suddenly it's spam for everyone using that system. It's annoying...but not having aggressive, shared spam filters would probably be more annoying in life.
So! If and when you don't get your Sheldon daily e-mail, check your computer-based or server-based spam filters. 99.99% of the time, that's the problem.
(Also: If there are Gmail or Yahoo engineers who'd like to offer up a more permanent solution, that'd be awesome.)