I've had a look at the PM, and the syntax for the Private Line looks fine - no extra spaces or special characters in the character name, f'rinstance, so I don't think it's anything you're doing wrong. Doesn't surprise me that you can see the Players' Private Lines as GM access lets you see all Private Lines (ie there's no checks) whereas Players can only see ones addressed to them, and I think that check is either broken or missing).
I did find a note in the FAQ (under
Background FAQ | What are some of RPoL's features? /help/index.cgi?t=faqs&page=backgroundfaq) that suggests that private lines only work in public messages:
Private Lines: Players and Game Masters can pass private notes to one another inside public messages.
However if private lines
aren't available in private messages, then the compose screen shouldn't include the option.
I can't see how it's anything to do with the GM posting in the 2 person thread, as that doesn't cause a new thread to start, as adding a new participant or removing one of the exising ones would, but you never know ...
I'll refer it to jase to have a look at when he gets a spare second. ;>