Probably because the cheat codes have already been set for you and probably because you already used the codes. Why not try using the shredder button on those unwanted codes?
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there is a master code cheat to use the cheats in the action replay with out it the codes wont work
shut the ds off put action replay in ds. turn on press action replay. take action replay out finally put ds game in and press on the star
they break the action replay so u can use it on diamond and pearl but if u put it on platinum it will break and wont work on anything....SO DONT USE ACTION REPLAY ON PLATINUM
If it comes up saying ''UNOWN GAME'' it means that your Action Replay hasn't got any cheat codes stored for that game. You need to aquire the games ID and then find the codes you want and then store them on your Action Replay.
www.supercheats.com/nintendods/pokemonpearlcodes2.htm Has every single action replay code for Pokemon pearl. You wont believe some of the stuff you can do with these codes!