The other day, I discovered a problem, and solved it today.
I wanted to print a manuscript in view of an edition, because I find it easier to work on printed documents than screen ones. It is just a matter of about 80 pages, but each page has a scan of the original page, hence the heavy files.
I created three files A, B and C, and converted them to PDF. A was large (41Mo). I used SmartFTP to upload them. The transmission was completed in about 5mn. Unfortunately they never appeared in my list of files. I concluded it was a problem of weight.
I divided A into two new files, A and B, and renamed the old B "C", and the old C "D".
This time the four files reached my Lulu folder.
Lesson: if the first file of a batch is too large, it won't be accepted by Lulu, and it will block the others.
A secondary problem appeared: my files were in the 21.60 x 27.90cm format, and Lulu warned me that they should be in the 20.99 x 29.70cm format. No big deal. I redimensioned them this afternoon, and my book is now ordered.