There are genuine times that preboarding "elites" is not wise, given the number of "elites" on the plane. My experience has been that when that happens, the agent consciously announces it. Most often the lack of a preboarding announcement for "elites" is through oversight as just happened to me LAX-DFW. Polite pushiness immediately after FC have started to dwindle but before coach rows are called seems to be a reasonable tactic. If I get met with a "not yet" I respond "well when?". Unless they have a reason to suspend the rules, the rules are the rules.