We have had a few users contact us in regards to not receiving Notifications on their devices or even tiles not updating in the background. Although it is unclear what is causing this issue, it would appear that it is more widespread than just our apps. The marvellous folks at wpcentral have reported that a few of their users and even them selves have suffered from this issue and have even posted a fix.
Basically the fix entails going into your phones settings and then into the Background Tasks option and turning off all your active background tasks. Then make sure you have set them all to be re-enabled the next time their app is run and this should fix the issue (it did for us).
You can see more detailed information and even a screenshot walkthrough in the original wpcentral post here.