sambul29, when TrackChecker gets a "Delivered" status from one of services added for track number it stops tracking that service.
But you can switch off auto status "Delivered" in App and manually remove status "Delivered" from track.
For that:
1. Switch off option: Settings -- Postal services -- Mark as delivered.
2. Open track number
3. From menu at top right coner select "Mark as not delivered"
TrackChecker will tracking that track again till you give them "Delivered" status manually.