2D Animated Book. Sprites don't render unless z value is around 500

Using a main camera set at z=-12, sprites don’t render unless their z value is > 474. They are not hidden behind the background (at least not visibly in 3d view) I have introduced sorting layers - background, middleground and foreground and assigned these to the sprites. They still disappear if z < 474. This is both in game view and scene view.
While setting the x axis values high makes it all work, I’d like to know why, if anyone can help. The book itself is available free on the Apple app store for iPad only. I’m working on an Android version. This works on 16:10 tablets but not on a phone. The background and some other sprites fail to render. I’m therefore wondering if this is connected to the z axis oddity. If you do look at the book, the zookeeper on the first page is an example of one that fails to render if z is set to <474.
Thanks,
Wal

Simple answer. My grandson spotted it. A game object which was the parent of all of these sprites had its z value set to just over 489. Not sure how it got there but correcting this to zero fixed the problems.