The most common cause of a missed recording is when a lot of timers are set and stored in a "bad" order within the Toppy—a bug causes the wrong information to be communicated to the wakeup system. This is fixed by the [T] (or [T2]) Timer firmware patch.
Another possibility is a power cut when the Toppy was in standby. In this case, the Toppy will (ordinarily) stay on standby when the power is restored. But it is likely that the Front Display will have lost the time and be reset to "00:00" at a date in the past. So it will not be able to wake the Toppy up at the correct time for the next recording. This issue can be fixed by the [P] PowerRestore firmware patch, which will cause the Toppy to turn itself on when the power is restored (and it will then set its clock properly, so future timer recordings will work).
Other causes are a crash (for various reasons, including a rogue TAP) or when the Front Display gets confused. Both should be signalled by the "Four Flashing Dots of Doom" on the Front Display.