My unscientific opinion is the upgrade rate has gotten worse the past 18 months to coincide with increased demand even if rooms go unsold that would be available for an upgrade… I also think some of the staff at the hotels are not “career” hospitality people and simply doing a job that was open 12 months ago and that has impacted the formality of being treated as a top tier elite…
But my success rate has been pretty good even with those noted factors (including many in Europe) probably because i choose to frequent the hotels i know are treating me as a loyal guest, which are predominately traditional Starwood portfolio hotels which to me always had a better upgrade rate than traditional Marriott portfolio.