I've had this problem before .
Make sure that the GBA games are real. There are ALOT of suspicious cartridges out there.
You can check if its a fake by these things :
-No Nintendo Seal of Quality
-A sort of "rough " texture to the cartridge sticker
-The logo looks a little.....unreal
(It is quite hard to explain but here is an awesome video)
Or it may be because your Platinum Version is on R4
I had friend who's Diamond version was on R4 and he couldn't migrate it but on the real cartridge he could.
This sounds a bit like your problem