Interesting, a couple weeks ago I ran out of Taq and got a vial of Taq expired in 2005 from next door and it didn't work. I put in some fresh DTT to 1mM and ban it worked nicely.

Enzymes could form incorrect SS bonds during storage resulting in loss of activity. DTT could leak over time from the Taq prep, getting some fresh DTT helps reduce the SS bonds and allows the protein to fold back into its correct conformation (energy minimum) and form the correct the SS bonds and regain its activity.

Take home - don't trash your expired enzymes just yet.