Help in programming question - Thread - Urban Exploration Resource

I'm in an intro to programming class, we are programming in scheme. One of our lab questions is to create an interative algorythom to exponentiate numbers using succsessive squaring, I think... So instead of writing a procedure to multiply x by x n times to solve x^n, I'm supposed to write one that recognizes that x^2n=x^n*x^n, which would therefore take less steps to complete.

I wrote the procedure so it checks if the number is divisable by 2, and if so, it divides n by 2, and adds one to shouldsquare, which is later used to square the result I get. If not, it multiplies the product so far by x, and subtracts 1 from n, when n=0, all the squaring takes place, and once that's done, a result is printed. But it don't work. Here are some sample cases:

STk> (exp 2 2)
8
STk> (exp 2 3)
16
STk> (exp 2 4)
32
STk> (exp 2 5)
64
STk> (exp 2 6)
64
STk> (exp 2 7)
128
STk> (exp 2 8)
512
STk> (exp 2 9)
1024
STk> (exp 2 10)
1024
STk> (exp 2 11)
2048
STk> (exp 2 12)
1024

- 2^6,10,12 fail. Same with 3^6,10,12

And ideas? I can post my code too if that would help. I think I just missed something simple, and I don't know why, but ^12 comes back the same as ^9, instead as same as ^11 which I think is weird.

(define (exp b n)
(expt-iter b n b 1))

(define (expt-iter b n sofar count)
(cond ((= count n) sofar)
((= n 0) 1)
((< count 2) (expt-iter b n (* sofar b) (+ count 1)))
((<= (+ count count) n) (expt-iter b n (* sofar sofar) (+ count count)))
(else (expt-iter b n (* sofar b) (+ count 1)))))