TRD Engagement Problems

Hullo all, I recently purchased a '99 w/ TRD and when I first tried locking the differential it worked fine (and climbed a nice hill!).

However, recently during a massive snow dump (live in Tahoe) I was going to see how far I could go in our neighbourhood B4 getting stuck. There was about 4 ft of snow and I made it about 1/2 block! High center anyone? haha. Well, the TRD light just kept flashing and not engaging. But I did get unstuck without it...

Anyone had this problem and/or have an idea what is causing it? I bought this truck because I wanted the TRD feature. Thanks, Rick

BTW: There is a LOT of great info on this site!