Depends what kind of cord your using. This is just for me I'm sure but I have to raise the 1P GBA to head level before the game sees the link. Another thing too, have you beaten both games? I think I heard you need to beat those games before you can trade (bet I'm wrong on this though).
You need to beat the Elite 4 in Fire Red, and fix Celios Network Machine.
It should say (If you haven't done this) You cannot trade with someone this far away (Or something like that.)
Before you can trade between FireRed and Emerald you ALSO must have beaten the Elite Four in Emerald and obtained the National Dex. (This is answered in the Emerald FAQ, by the way.) Is this your problem?