nikon – missing one image in sequence

Shooting with a Nikon D810 on a 64GB compact flash (Scandisk Extreme) with an 32GBSD card (Scandisk Extreme Pro) backup (camera set to write to both)
After photographing a scene, and in review of the cards, the first 2 image were on the compact flash card but missing from the SD card, and a random image in the sequence (image number 34 in a sequence of 60 images) was missing from the compact flash card but on the SD card. No errors appeared at the time of shooting and cards were formatted in camera prior to shooting.
What can cause this to happen? and how do I assure this doesn’t happen again in the future?
(these are criminal scenes and crucial to be capture securely and in sequence)
Thank you