I've had this happen a few times. I was puzzled at first and now I simply move on. I think some may want a donation to unlock, some may want more of a message (showing genuine interest) than the standard RM "please unlock". I'm sure they get unlock messages all day long (photo collectors) which never go anywhere, some may simply decide it's not worth their time to respond if there's not more of an expression of interest. If I'm genuinely interested in someone, I'll try to craft a 1-2 sentence email with my unlock request.