I'm having problems adding only certain contacts. Some of them work just fine; I add them or they add me and we talk normally. Others, however, have different results.

One person added me on her msn. She can see me and message me, but she doesn't show up on my friends list at all. Neither online nor offline. It says that she doesn't have Windows Live when she clearly does. And when she messages me, it says offline. To message her back I need to log onto the Windows Live People.

The other person I'm having problems with isn't receiving my invites, and I'm not receiving hers. She shows up in my offline on both my Messenger and on Windows Live People, but she's offline and we can't message each other.

I know there are several questions exactly like this, but I haven't found any solutions that fix this for me. If there's anyone that could help, it'd be greatly appreciated.

Well unfortunately we aren't Microsoft and therefore your best bet would probably to seek answers from them regarding this. As it works fine with other contacts...

Can you see and chat with them fine on a web based messenger such as http://ebuddy.com?

I can only see the first girl on ebuddy.com or elsewhere. But I can see her if I log onto the Windows Live People. However, I still can't send/receive requests or communicate with the other girl on my Windows Live Messenger. She's just in my offline section, but she says there isn't a request; and vice versa.