Moov counts your reps when you complete the correct movement for that exercise. Reps are recognized based on how you move and if you are completing the correct motion for that exercise. Follow the coach to see how your arms should be moving in order for your reps to be counted correctly (e.g. swing your arms up when you squat). The coach in the video is moving in the way that accurately counts your reps.
If Moov is still not counting your reps, use of Apple Airplay or Bluetooth speakers may be interfering with the connection and turning off these devices might help.