Quick Tools customisation in Acrobat DC

Hi everyone

Right-clicking in the Quick Tools toolbar allows me to add preferred icons for frequently used tools, or to remove those that I rarely use.  It then allows me to save the changes but when I close the program, it apparently does not save them because when I then re-open, my changes are gone and the default icons return.

When I do the same with the "Show Recently Used Tools" option however, the program remembers and they are displayed.

What am I doing wrong?

Thanks in advance for replies.

Derek