On the Bronze level, when the data size is $N=100$, there is no significant difference between $O(N^2)$ and $O(N^3)$.  It's like 0.01 second and 1 second, you notice the difference in speed, but it won't be that much.  When $N=10^5$ or something like that, it will be quite different.

To find the bug in your program, print out some more intermediate results to debug (or use the debugger in your IDE).