It is well established that shellac dissolved in alcohol ('liquid shellac') has a limited shelf life. The actual shelf life is highly variable, depending on the source, manufacture, storage. But after sufficient time, a batch of liquid shellac will no longer dry hard. A good test is putting a little shellac on a piece of glass. Fresh liquid shellac will dry hard in about 15 minutes. Sufficiently old shellac will stay tacky/gummy indefinitely.
What is the mechanism behind this change in properties?