I have the same problem. I find that right clicking and dragging the item from my equipment bar of my character screen into an empty spot on my HUD equipment bar will fix it.
Or, I noticed that when it does get equipped it does not always show up in the spot you think. Expand your HUD's equipment bar to show more lines and you may find that it was equipped to a different number spot. If that's the case just drag it to where you want it to be.