Did you train at a Myzone gym?
If you have small or large gaps in your graph, it is likely that your facility's clock is set incorrectly on the in-club system. Let a staff member know ASAP so they can correct the issue on-site, and try keeping your mobile app closed while at the gym to prevent discrepancies when possible.
This means either your belt did not record your heart rate accurately or some of your workout was dropped due to a network error. Please see Missing Workout: Not Performed at a Myzone Gym