By default, Watch OS will revert back to the clock face if the app is not actively being used. You can override this setting using the Watch app on your iPhone. This will keep the Full Swing app open on the watch for up to 1 hour.
1. Open the Watch app on the iPhone.
2. Tap General, then tap Wake Screen.
3. In the APPS section, tap the Full Swing app.
4. In the Return to Clock sections, tap Custom, then tap After 1 hour.