There are lots of reasons why a particular incident might not have been caught on BWC.
1. The officer failed to activate it. The most common model of BWC (Axon) requires a double touch to the main button to start, if the camera power switch is already on.
2. The camera was out of power. Unless they are regularly charged, a typical BWC won't last an entire shift.
3. The camera was faulty. There are known issues with batteries on some BWC, they will often run out of power in a short while even if fully charges.
4. The officer's camera was faulted, turned in for repair.
5. He might not have been issued a camera. The investment for giving every officer a BWC and the associated docking stations that require VPN connectivity, and expensive server storage, is a lot, particularly for a small department.
Most departments with BWCs require officers to activate the camera at the beginning of every public interaction, not just when things get active. Turning on the camera after a shooting is pretty useless for evidentiary purposes.
If this officer didn't capture the incident on his BWC, other officers with him would have captured it on theirs. Any serious incident investigation would look at the BWC and available CCTV footage from every source, police and private.