I cant connect to some people on xbox live?

I can connect to almost all my friends but 3 or 4 of them i can never connect and the only way to even talk is to be in the same party just with someone else as the host... this is mad annoying why does this happen and how can you fix it? dont say unplug any thing because we have tried a million times so yeah..I cant connect to some people on xbox live?
i had the same problem with a d-link router. the only way i fixed it was to get a new one. sorry.

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