EPub Acceptance in IngramSpark EBook Review

My EBook project has compiled fine and looks great in Adobe DIgital Editions 4.0.

It also passes muster with validation (validator.idpf.org/application/validate).

I have been through the EPUB GUIDELINES documentation for IngramSparks numerous times. Still, my file is rejected after being accepted in the initial technical review.

I am missing something.

Does anyone have any words of wisdom to offer here?

Stephen