To my knowledge there's some unknowns as to why we can't always browse someone. Even if they have the chat enabled. And you're right, if they have a yellow chat icon they are much easier to browse. It might be due to you or them not having enough available bandwidth or something else. Just b/c you can't browse them doesn't mean they're not sharing. Personally I have my Preferencing set to a minimum of 30 files (Tools>Options>Advanced>Preferencing.) I presume this works. I do know the slider doesn't work.