Well, I think there are two possible reasons why you are scoring lower each time. Well, 3 I guess if it's just purely random. But the two reasons would be that either stress/anxiety is getting the better of you (and of course it will be worse each time) or you are not studying effectively.
So, managing stress is definitely a big part of doing well on the exam. If you went in the first time with no expectations, then it probably wasn't an issue. If you put in a lot of effort studying for the re-take, the value of your time and effort may cause you to put additional pressure on yourself to do well. You can read some of the reports around here about how people have managed their stress. It definitely plays a big role.
The second potential problem is that you are not studying effectively. You say in your message that you are preparing for the exam, but as most of us here know, you need to make sure you are learning rather than just doing problems. So, some people like to use an
error log to identify the problems that they miss; then they target their study towards that type of problem. Others have concluded that they need to improve their verbal or math foundations. You can't learn new material until you are 100% solid with the basics. Some people go back to high school math books and English grammar books to re-learn the basics. Once they have those cold, they can deal with the challenges of the GMAT.
So, make sure that you are studying effectively. Figure out what you don't understand and target those areas. Then, as you get closer to the exam, work out a set schedule so you can simulate the actual event and manage your stress.