Mine were flaky at best before the Black update and have worked perfectly after the Black update.
Before Black update, I could only support 3 total live tiles, and even with 3 they weren't updating.
Keep in mind, WP can only update live tiles every 30 minutes (I believe that's the case).
Maybe there's a cache somewhere that gets corrupted. Clear all caches etc.
When my Bing Weather app crapped out on me before Black, I would uninstall it, restart the phone and re-install the app and everything would work perfectly.