It is my understanding (from personal experiences), that its possible you can not be added to an upgrade list if it is already pretty full and you stand no chance of an upgrade. I had this happen at DCA when I was rerouted from a delayed IAD departure. Got to the gate, didn't see my name, asked why not, agent tried multiple times to add me, and I was finally told that I had no shot at the upgrade and that the upgrade list was not allowing me to be added because of that.