Pdf/a isn't working on big files?

Hi

Can someone help me?

I tried -PDF24 creator 10.7.1- to convert some normal pdf files to PDF/A-1 and some of them were succesfully, but for example this file won't be converted. When I open it with adobe reader ther is not blue line on the top.

This is the original:

https://files.fm/f/k4qjxd63n

This should be pdf/a:

https://files.fm/u/9sjxaw99w

What are the limitations for converting to pdf/a?

Thanks