Just Zones reads your workouts and heart-rate data from Apple Health to build your weekly time-in-zone charts. Your Health data is read and processed on your iPhone; it isn't uploaded to any server of ours (only the first point of a workout route is sent to Apple Maps, to name the place), and Just Zones never writes anything back to Health.
Just Zones needs Health access to show anything beyond sample data. If you denied it by mistake, go to Settings → Privacy & Security → Health → Just Zones on your iPhone and turn the categories back on.
Open Settings inside the app and go to Zone Setup. Just Zones can estimate your zones from percent of max heart rate, from heart-rate reserve (Karvonen formula), or from lactate threshold heart rate (Friel), and you can also set fully custom zone bounds by hand. The app always shows which model produced the zones on screen. See how zones are calculated for the formulas.
Just Zones only assigns a zone to time that has a heart-rate sample from Apple Health to back it up. A workout with gaps in heart-rate coverage — a slipped strap, a paused sensor, a stretch with no recorded samples — will show less total zone time than the workout's duration, because that gap isn't guessed into a zone. Readings below your Zone 1 floor are excluded too: that time isn't assigned to any zone, so it doesn't count toward the zone minutes.
Just Zones reads workouts straight from Apple Health, so a missing workout usually means it isn't in Health either, or Health access for workouts was off when it happened. Check Settings → Privacy & Security → Health → Just Zones, and confirm the workout appears in the Health app itself.
Percent of max heart rate, heart-rate reserve (Karvonen), and lactate threshold heart rate (Friel) each define zone boundaries differently, so the same heart rate can land in a different zone depending on which model and inputs (max HR, resting HR, LTHR, age) you've set. See how zones are calculated for a worked example.
Open Settings inside the app and tap Restore Purchases. Pro is a one-time purchase with Family Sharing enabled, so it can also unlock automatically for family members who share purchases.