why does it takes days, weeks even for a PM to leave my box?
You didn't specify if this is this every private message you send, or just maybe one or so. Naturally I'm guessing the latter, but someone should ask anyway.
In the case of people not opening messages, there are two main scenarios that come to mind:
1) The recipient is new to the boards and hasn't yet noticed the notification that there is a message for him/her. That happened to me as a recipient when I was new here.
2) The recipient is a member who is either not that present, or generally doesn't log in every time s/he reads the Chiffboards (whenever they do stop by for a read). You can't see a personal notification without logging in. Related to this, the recipient could be just a "drive-by", as we call it. A wave and a hello, and then it's toodle-oo Chiffers, it's been grand but I must be gone now and on my way to elsewhere bump my Google ratings. Have a nice life, all.
With the above, basically the best guess is they don't know they have messages, either because they don't or can't see that they have them when they're here, or not here, in which case, well...they're not here. Guess that's what the emails are for.
but they can see who it is from
Okay, three scenarios, then.