I'm not so sure about the problem, but I think it can be something about their privacy setting. For example, if your friends been tagged in the pictures but you were not friend with your friend's friend, you might not be able to view their pictures doh..
Problem: I'm unable to view all of my friends photos that they have been tagged in on facebook.
Request: Does anyone know;
a) Why this is so?
b) How i can view these photos?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic