Strike-thru shortcut

When marking up PDFs, I used to be able to highlight text and hit the back key to mark a strike-thru. Now, I can only do that once. After the first time, I have to use the menu that comes up after I highlight something. Oddly, if I click outside the application or into another document I have open and come back, I am now able to use the back key again, but only once.

Why is this happening? Is it possible to make the back key work all of the time?