Where are you in the game? There are a couple places they are disabled, like the final room under the catacombs.
When you enter the castle an imp will 'borrow' them, if you have one on you. If you don't, he will show up the first time you try to use one after getting out of the castle, or they stop working until the point you would have got them back.

Depending on what the problem is, a couple people have got them working by placing the set they have in a barrel somewhere and downloading the inventory files for a new set (or getting them from a previous save of their own).
Lost Items - guide and directory of inventory files

Since you are about to enter the castle, I would just bring one/both with you. Their status will be changed and then changed back as part of the plot, which I would expect to get them working again.


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