In the past 5 years and having flown not far off 2000TP per year every year - (80 in old money) I've had a grand total of 1 surprise "status upgrade" if that's indeed what it was (truth be told I didn't ask and didn't care).

I was Gold FC at the time, I paid for PE, It wasn't a late booking, the PE cabin wasn't full (nor was UC where I was moved to), oh and I always ask!!!

It was a very very lovely surprise on an 11 hr LHR to LAX flight

So no, I doubt you're doing anything wrong, these upgrades are just a very VERY rare occurrence!