Yea i know man that sucks too. You'll get used to it, i barely changed it today too! (Started playing last night)

This was happening to me too. If you are using the default keys (the F1-F5 keys) then it wont let you do more than 2 keys at once. You have to change the key settings. I changed the keys to be by the numbers, starting from 1 to 5 or whatever you want.