This is a known problem with Adobe Acrobat PRO DC - https://forums.adobe.com/message/9035518#9035518
I also experience this problem with abracadabraTools.
This is with Adobe Acrobat Pro DC 2015 Release (Classic) Version 2015.006.30394 on Windows 10 Pro with all updates applied.
After restarting Adobe Acrobat my toolbar looks like this - as you can see no QuickTool icon for any AbracadabraPDF addon is visible
But, opening Customize QuickTools, *without* editing, shows the Flattening Icon is *already* included in the QuickTools tool bar!
To make the Flattening icon appear in the QuickTools toolbar I simple select it in the Customize QuickTools window and use the left/right arrows (top right of this dialog) to move it left and back right and then click "OK".
Then the Flattening icon appears in the QuickTools toolbar
But when I quit from Acrobat - its back to the first screenshot above - no Flattening icon is visible!
Can you fix this, or ask Adobe to fix it if its their problem?
(Sorry I couldn't figure out how to shrink the screenshots above - you have to scroll left/right to see them)