Hey there!
So I was having the same problem of being unable to transfer my notability files from one device to another.
This is how I managed to fix my problem:
(My notes where on my iPad and I wanted them to transfer to my MacBook)
1) If you have an iPad - go to the the gear on the bottom left of the page, select icloud, flip the switch to ON. If you do this, it should announce that your files are being transferred.
2) If you have a Mac computer - open the notability app, select the notability tab on the top left corner of the screen, select preferences, a little screen should pop up and at the bottom there will be the option to turn on your icloud.
I would recommend turning on your icloud on whichever device has your files FIRST, then turn on icloud for the other secondary devices after! If you were like me and did it backwards, no problem! Just turn on icloud on the device which has the files, then go back to the other device turn icloud off and when the pop up screen asks you to keep using icloud, select that! All your files should be there afterwards!
Hope this helped!!
-K