I've wondered about this. For an upcoming flight (still 7 weeks out) I was getting upgrade offers to D1 for quite a while... Until I got hit with a Saturday schedule change. Nothing crazy, and I accepted (and continue to when it happens) but the offers disappeared and I've not seen once since. I've wondered if the banner was supposed to go away, but it appears to 'stick'.
Would there be anyway to call to see if I should be getting offers or not? I don't have any status so I don't imagine I'd get a lot of help with regular CS...