I can understand the part where their status says "Pm me if you need help." and they have friends only on. That can be irritating. However, they do get swarmed with messages and a lot of them are most likely "Hi" "Can i haz hat?" "this guy is hacker!" pointless pms like that. Not responding can be because of one of the following reasons:
1. They're AFK.
2. They're busy and can't respond to pms at the time.
3. They're spammed and accidently skipped your pm.
4. You're pm is pointless.