Two accounts-one work--one work not

Hey

i have a question:
I have 2 Modern warfare 2 Keys..one from original Game and one from a Key store where i buy only the key;-)you know what i mean!! Since two days i can´t join to a game with the one from Keystore..before no problems with this over a month!! The other original have no problems to join but there i don´t use Hacks...Both keys are not banned or so...pls can anyone help me??