I have noticed that this tends to happen on hard shots if the coupling between the cue and the DigiCue unit is too loose. What happens is that a hard shot (or sometimes even any shot if loose enough) is attenuated by the gap, or a sliding rubber bumper, and doesn't trigger the stroke detector circuitry. This can be fixed by using the smaller of the two rubber bumpers and making sure that the plastic case of the DigiCue is in contact with the bumper of the cue (even if it fits a little tight)
If you are already using the small rubber bumper and you still see intermittent missed shots, then it could be that the DigiCue is infact triggering properly but the radio signal is being attenuated as seen by your mobile Bluetooth device. Metal surfaces tend to cause Bluetooth signal issues. You can test to see if this is the case by Custom setting a vibration on both pass and fail. You can then check to see if the app reads data every time the DigiCue vibrates. If you still see issues, it could be something in the app itself, which can be tested running two simultaneous smart phones and seeing if they both receive data at the same time.
Thanks for the info. I am sure it is not a case of bluetooth signal because it ONLY happens with a hard strike, not with a soft strike from the same position. I will try to adjust it to fit better.