It's like this. It depends on the situation. If they're ones who are likely to respond to PM's, then they will. If not, then they won't.
I wonder why you are sending PM's to people who don't respond. I know you understand, once is enough, but please do not complain.
Sometimes, people do not comprehend the nature of your messages. Trust me, it's happened to me many times before, most recently with the young lady who I was considering inviting to join this forum.
However, I'll tell you something. She used to be a social worker, and she is one smart cookie. Even she would not tolerate anyone doing this kind of thing.
As the saying goes, "All good things come to those who wait." Trust me, please!