Create Bookmark keyboard shortcut (CTRL B) doesn't work in all files

Dear all,

I have problems in ACROBAT PRO DC with keyboard shortcut for CREATING BOOKMARKS in some PDF files. The CTRL-B shortcut doenst work in some files (while I can create the bookmark via the respective button on the bookmark pane), while in others it does. This is not about file restrictions as I can always create bookmarks via the side pane by clicking on the respective icon!

As I create bookmarks all the time, a keyboard shortcut is highly necessary and I wonder why in some files it doesnt work..... can you help?

I use Acrobat Pro DC 15.006 with latest updates installed on a Windows 7 PRofessional 64bit system.

Best,

Erik