what's wrong with my response.xls file?

Hello,
I'm seeking help on the response.xls.
What I want to do seems quite simple: It is a categorization learning experiment. For each trial, the subject answers two questions (each with two options); then, based on the answers (so there should be 2 x 2 = 4 possible answers for the two questions), the computer will give feedbacks (so there are, again, 4 types of feedbacks to the answers).

The four feedbacks are named as f2, f3, f6, and f7.
I used IF and AND functions of Excel in the response.xls file:

=IF(AND(B1=1,B2=1),"f3",IF(AND(B1=1,B2=2),"f2",IF(AND(B1=2,B2=1),"f7","f6")))

the response.xls looks like:
(the b1n1c and b1n1d are the two questions)
--------------------------------------------------
variable value skipto b1n1c 1
b1n1d 1 f6
f2

f3

f6

f7

----------------------------------------------------

I cannot see any problem with my files, but I do not get what I expected to get. Instead of giving correct skipto patterns (and thus feedbacks), no matter what the answers are, the Medialab just continues with f2, f3, f6, and f7, in this order.

What am I doing wrong? Much thanks!!