iCloud sometimes does not upload certain files (on iOS). It is not known what causes this. Other newer files continue to be upload, even when added later.
This is a serious problem to the framework, because on other devices, iCloud locks the file that hasn't been uploaded, and the sync process gets blocked completely.
In an attempt to help remedy this, a check is now done in the file manager app sync manager. It monitors un-uploaded files, and if one stays unuploaded for a certain period of time, it copies it out of iCloud, removes it, and moves the copy back in again (with file coordinators, of course). It is thought this will 'jolt' iCloud to notice and upload the file's copy, though it is very difficult to test, because you need to wait for the situation to arise.