I've just spent a couple of days wrestling with the above issue, and was beginning to dread the fact that it might be an problem with V1 Affinity products - which they would be unlikely to want to address. But I found what I believe is a useful work around.
File type, colour profile and resolution are obvious issues that Ingram-Spark's servers rigidly enforce, but that would apply regardless of the originating application. I loathe Adobe's extortionate business model and refuse to use their products, so I was worried when Ingram-Spark required an Acrobat compatible PDF/X-1a:2001 codec, thinking that might be the issue. However I learned that PDF/X-1a:2003 (available in Affinity Publisher) was perfectly acceptable to the IS server, and what I thought was a compatibility problem is more likely to be a time-out issue with their server. As someone else mentioned, I have also had revisions to my book (the text file is 650mb) accepted on at least one occasion, but mostly it hangs at about the 80% mark (despite them saying file sizes of <1.5gb are acceptable, they clearly aren't'!). So the solution is a faster connection if you can find one, or as I discovered, a smaller file which uploads quicker. Preferably one under 500mb.
I'm a Mac user, so the compressor I found was on the App Store, called PDFOptim. It is excellent, very affordable, a purchase to own product, it allows fine granularity in what/how much compression it applies. It allowed me to bring my file down to around 380mb with less that 5% image compression and no discernible loss of quality on the page. My book is a cookbook with a ton of large image files, so I was nervous about what they'd look like after reduction in size, but PDFOptim did the job and LS accepted the document first time.
Hope that saves someone else a couple of days of their life.
