The basic reason for delay in Ielts test is if the difference between applicant;s scores for the two sections is higher than 2, this makes the Candidate's test withheld and handed over to the different examiners to remark the test. In test re-evaluation process, it takes around 2 weeks, then only result comes out, that's why IELTS test result get delayed.
One of my acquaintance informed me the reason why IELTS test result is delayed, if that is what you are asking. He said if the difference in a candidate’s scores for two segments (lets say Writing and Speaking) is higher than 2 (again lets say your scores are, Writing:8, Speaking:5).
If this happens the candidate’s test will be withheld and given to different examiners to remark. This is because, although it is different skills being assessed, one is not expected to do too good in one skill and then do too bad in another. This process takes roughly 2 weeks and the result will be out.