Wierd behavior of gc.collect

[Bodhi]

I have a python process that does some operations and is supposed to release memory after those. The issue is that memory is not released (as seen through top). So I do a gc.collect() to see if there is any cycle etc. Immediately after doing the collect memory usage drops as expected, but strangely gc.collect() returns 0.
This means I cannot find out what the problem is by setting the debug option on gc which is what I usually do to figure out issues like this.

Maybe its that my understanding about it is incorrect, but if gc.collect returned 0, how come some memory was freed?