I recently unearthed a ~30 year old jar of Behlen shellac flakes (stored dry, in the dark, they were still loose flakes). I tried hard to get them dissolved, pulverizing them first, soaking in solvent for days, finally running the rubbery globs through a strainer and trying heat to get them to dissolve. (After it became clear they weren't going to dissolve readily I just wanted to get them in solution so I could use them to make "burnt shellac", a rather remarkable adhesive used to make reversible airtight wood/metal seals in player piano work). After two weeks of effort I gave up and dumped the mess.
I have to assume that there is some level of polymerization that proceeds over time that eventually renders the shellac unusable. This stuff was OK ~15 years ago, the last time I remember using it, before it got lost in the move. I was surprised because I'd always thought the shelf life of dry shellac was indefinite. This was orange shellac, a second bottle of "super blonde" of similar age went into solution as usual and behaved perfectly; so perhaps the level of impurities affects the shelf life.