I assume you got the first 2,400? Did you enter the first 2,400 in the first box and enter a 0 in the second box?
What you may be seeing is that Turbo Tax started out assuming you didn't get any Stimulus payments so it gave you the full 3,600 credit. Then you entered you got the first 2,400 and the credit was reduced to 1,200 on 1040 line 30. That reduced the initial refund you saw or increased the tax due. Because you already got some.
