I was having the same problem. I am using Windows on the desktop. The drag and drop method was not working as stated in the help instructions. I found that after selecting the notebook I wanted to stack, I needed to click on the 3 dots (more actions) menu above and to the right of the notebook name next to the filter icon. Then select "Add to Stack" from the list.
Moving notebooks into Stacks
in Evernote for Mac Issues (Versions 7.14 and under)
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I was having the same problem. I am using Windows on the desktop. The drag and drop method was not working as stated in the help instructions. I found that after selecting the notebook I wanted to stack, I needed to click on the 3 dots (more actions) menu above and to the right of the notebook name next to the filter icon. Then select "Add to Stack" from the list.