So were you plugged into PA announcements? Often the PA system through headphones is not switched on early enough. Were you listening to music through the headphones? The noise cancelling feature would be attenuating the muffled PA voice trying to get through the headphones to you! Far better just to do as you're told and start using the things after take-off. The cabin crew don't have time to go round shaking every individual shoulder or inspecting every handset to see if it is in flight mode or 'yes, I know it's only a GPS (or MP3 player), but would you turn the damn thing off anyway please! Before I get angry!'