Sika are very wary and I've had an idea that the big stags, especially, simply avoid the trail camera. Even hinds will usually carefully walk in front of it the first time and then simply not come back and so over time the number of photos can fall away. Anything out of place and the big stags don't ask questions, they just leave and don't come back. The camera is relatively small and is attached pretty low down on the tree with nothing to make it stand out at all and at this time of year it is rarely dark so the flash doesn't fire much to annoy them so I'd have thought all was good, but that is not the case.
I've had a few "white out" photos were I was pretty certain it was a nose giving the camera a good sniff and close examination but this time the camera had been out for a long time and so might have had very little human scent plus he wasn't a very big stag and so wasn't as cautious as a bigger one.
The first photo shows what I believe to be a nose having a good sniff at the camera, I've had similar photos where you can actually see the whiskers. About 10 seconds later the deer appears.