If fixture in a zone with other fixtures stops working, the IJB is sending the signal to turn on. If it turns on just one the IJB is good.
Check the following:
1. Make sure all the lamps are the same make and wattage. Inexpensive foreign lamps can sometimes cause problems.
2. If all are correct contact the company who made the fixture for possible ballast or fixture issues (i.e. Peerless, Hydrel, Lithonia, etc.)
3. If a fixture remains on all the time, data communication between the IJB and the ballast has been lost. Check the continuity of the 2 purple data wires at the fixture. I have seen the insulation pushed under the knife edge connection in a push pin type wire nut.