Hello everyone, is there any guides that I can read on regarding playing multiplayer games between 2 PSPs?

Both are custom firmware 5.0M33 (both are fat PSPs).
One of the PSP (lets call it white PSP) can host games and join games.
However, the other one (lets call it black PSP) cannot host games or join games.

I have checked the side button for the network; both are in the right place.
The problem just happened recently (a week ago); before this, we can play successful multiplayer games everytime.