This came up once previously in the past two months, but its significance went right over my head (I have an excuse - I'm short).

Sending someone PM by clicking the "envelope" icon next to their name doesn't mean they'll EVER realize you've sent it. The message is delivered if they're a VIP member, but the sender has no way of KNOWING whether the recipient has that status. (Actually, the message apparently is filed safely away in the recipient's inbox, but the recipient has no way of knowing it's there, or reading it, unless he/she becomes a VIP member.

So, if you think someone has been ignoring you, it's quite possible he or she hasn't had the opportunity

members.  They just must be VIP members to read their mail.