1. Premier Exec 1K is just another term for 1K. Nothing more meaningful about it, just a name that they gave it and is sometimes used, sometimes not depending on what the computer spits out .

2. Your success in upgrading depends on many things, which is sometimes frustrating, yes. If you bought your expensive ticket very close to departure or were not added to the waitlist until very late (you know you can do this in advance, right), there might have been very few upgrade seats left if others had already been upgraded in advance.

On the day of departure, upgrades are given in order of: status (GS, 1K, PremExec, Premier, etc), then fare paid, and then time added to waitlist. So a GS member could be ahead of you.

If you provide more details about your flight and ticket, maybe others could give you more information, and you could use this to be more successful next time!