Take the breaker out and compare the back of it to the one you took out. Pay close attention to the length of the plastic slot where the buss bar tab goes in. It's probably too short, a common problem when mixing different makes of boxes and breakers as some buss bar tabs are a little wider than others. Square D, Siemens, GE, etc. Take a dremmel wheel and trim a little of the plastic away to make the slot a little longer, 1/8" or so to match the one you took out. You may have to lengthen the depth of the plastic slot on the side of the breaker as well to get it to go all the way in. Look carefully and you may be able to see where it is hitting the buss bar tab.
Note: In some areas insurance companies are getting picky about mixing components. I have heard of them balking on claims if a fire occurs, an electrical problem is the source AND there are mixed brand components in the main box. ie.. GE box Square D breakers etc... even though they all meet the proper spec.
