Are you adding the buffer to solution or solution to the buffer? Anyway, whatever two solutions are you mixing, the pH will be something in between the original pH of the original solutions (let’s say, you will mix solution of pH 3 and solution of pH 7, so you will get something between 3 and 7, depending on amount, buffering capacity etc. of the original solutions;)
Well, you are only adding salt of weak acid, so you should always get solution of pH in between of pKa1 and pKa2, unless you won’t add some alkaline (in this case add NaOH to make bicarbonate buffer)