Sometimes you lose a piece of equipment somehow.
It shows up in your blacksmithqueue, but you can't find it.
One way that worked for me was the following:

Go to one of your heroes that are in your formation.
Take the similar piece of equipment you are missing from that hero and put it in your bankspace.
Drop a new - unequiped - hero in your formation on top of the one you removed equip from.
The missing equip should be shown on the new hero you just dropped in formation.