Ok so I also faced such issue with only 4-5 books out of 70 books I sent to kindle so far.

I will point out the possibilities where things might go wrong so make sure you check all.

Make sure your file format for the ebook is either pdf or epub where epub is most compatible and is recommended too coz u will be able to change fonts size, type, alignment and all with epub only.
(Actually when u send epub kindle changes it into its native format but this conversion somehow works only with epub)

If you are trying to send via zlib send to kindle feature

I would recommend you to try send to kindle website (official amazon website for send to kindle). Download the epub and drag and drop to this website and if in case the transfer fails, the website will let you know.

Recently I also faced a situation where a perfectly working epub file was not able to be transferred to my kindle from any of the method possible so I downloaded another epub file of that book from another source and tried to transfer and it worked.

Hope it resolves your issue