browsing other's files

I seem to be having a problem recently connecting with certain other users to browse files. For most it seems to work fine, but with certain ones, it simply won't show up or I get an error message saying unable to connect to the user. I don't know if this is some configuration issue, though if it was, wouldn't it be an all or nothing problem, so it either works for everyone or no one? I did get an upgrade in the modem recently from the cable company, so would this change have any effect?