We are aware of an issue that is affecting some users on older iPhone models such as SE, 6, and 7. When the App is backgrounded for an extended period of time, in some circumstances, new messages, calls, or emails will not "wake-up" the App and notifications are not received.
We are currently working hard to resolve these problems and hope to have a fix for these notification issues as soon as possible. If you are affected by these issues, please contact support at firstname.lastname@example.org and/or follow us at Twitter so we can quickly inform you when we have a fix available for download.
We do understand this is a frustrating issue and hope to have a fix available ASAP.