Thanks for answering.
Actually, every font was embedded--PDF/X-1a ensures that--and the files passed every individual text related preflight in Acrobat Pro. CreateSpace acknowledged that. Their first answer:
Our team has advised that the conversion error is a result of the way the fonts were embedded. Indeed the fonts were embedded but as different types, for example, Type 1, TrueType, Composite TT. We simply run them through a process that re-embeds all the fonts as the same type.
Their second answer for the corrected PDF:
Also to follow up with your question in what our Technical Service Team did to fix your files was that they converted the interior to outlines.
Since, I need to make a few more corrections, and I want to avoid problems, I thought I would ask here.
Walton