Andy - you may find you're eating something that doesn't really agree with you, but doesn't make you feel unwell.
I do recall that when @Brunneria went gluten free her blood sugars dropped, even though she wa eating materially the same things. Ever carbier things (I recall talk of scones versus gf scones) returned lower scores.
I'm not suggesting you immediately go gluten free, but would ask you to think if you have any given food you like a lot and are drawn to eat a lot of? Sometimes we can be almost addicted to the things that don't do us any favours.