Hello! Bug time!
So it seems that altering the stockpile, i.e. changing what can be put in one, makes the use of class items completely impossible. So what do I mean by this?
Create a standard, all-encompassing stockpile, and the workers will move all tools into it (in the beginning of the game, and otherwise). Now, if you go about and change any of the contents that can be put in the stockpile, you cannot touch the tools, unless they are moved into a new, default stockpile. Here’s a couple pictures to illustrate it, albeit poorly:
Here we have a knife, perfectly usable at my call. But, if I remove the ability for resources to be put in the stockpile (or literally change it in any way), you cannot toggle the knife anymore, until it’s moved to a new stockpile (by removing the current one, or hoping that the workers move it to a new one you create).
In the last picture I have my mouse hovering over the knife, but sadly, screencapping with steam doesn’t include the cursor. But believe me, it’s there.
You can avoid this by simply having the stockpile unaltered, or by making a separate one which is unaltered.