The letter will tell you the reason(s) for the decline. Are either of your existing card accounts currently reporting a balance? I am not asking if you are carrying a balance, but if you had a balance to pay on the most recent statement(s). Citi may reject an application if they see no balance on any reported account.
Yes, one of my card carries a balance at the moment, but I always pay it off by the statement date. Why would Citi reject an app if they see no balance on an account? Isn't that a good thing?