Item not working after switching inv slots with 1-9

I made a custom bow; everything works great, but for some reason after switching the inventory slot that the bow is in using the number keys 1-9, it will not fire an arrow until I click on it (i.e. move even just back into the same slot) with the mouse, after which it functions again normally.

Even more strange is that it correctly nocks an arrow and is set in use, but simply fails to fire the arrow.

Here is the bow's code, and here is a step-by-step of what I did:

0. shoot bow, works great

1. open player inventory, highlight custom bow in action bar

2. press any number key, changing the bow's action bar slot to that index

3. scroll over to the bow with the mouse wheel or by pressing the number key again

4. right-click to use; arrow shows as nocked and animation / icon plays correctly

5. release right-click: no arrow is fired

6. left-click on bow, place back in slot, works just fine