What do you mean with "not send the invite"? Does it just not say anything and the person doesn't get an invite? That can be (and most of the time is) due to zoning.

If you mean that you get a random error message even though you should be able to invite them, I can confirm this, and what's worse is that I cannot find any pattern to it at all. Sometimes it works, other times it doesn't. Sometimes it will erroneously say "That person is a party member." even though they're standing right in front of you with their flags up. Sometimes it just says "You can't invite that person to your party at this time." or sometimes the very uninformative "That request cannot be processed.", none of which are particularly helpful. However, when you find people by searching for them and then send an invite from the menu, it always works.

This is one of the things that have always bothered me greatly, but they've been around so long that I got used to working around them and almost forgot about it by now. I'd love to see this addressed.