This has happened to me several times and the explanation I have gotten from DL is that they do not release all first seats for upgrade.
I guess in an attempt to sell them before the flight? Depending on your status if the seats are still available before takeoff you should receive your upgrade at the gate. Good luck!
I know if you do a search on upgrades not clearing you will probably get a more technical explanation... Here is a related thread:
http://www.flyertalk.com/forum/delta...1st-class.html