There could be several reasons for not debugging the entire bug in the first round of testing process. Debugging the showstopper in the first or second build is almost impossible. A found defect can cover up the other defects in the application. The thread which leads to on defect could be redirected to another defect, as the tester find the bug and lock that bug in report and after fixing of those bugs new bugs may also arises. It is difficult to keep testing on a known defective application. That is the reason we cannot find all the bug in first run and also we cannot perform Exhaustive testing.