Hi manjubhasin2058,
Practice CATs are really just "measuring devices" - in the same way that standing on a scale will measure your weight. To change your weight, you have to do MORE than just stand on the scale. To improve your GMAT score, you need to practice tactics and work on your "mechanics" to do the work properly.
As a general rule, I tend to recommend that Test Takers complete one FULL-LENGTH CAT per week. Most of your study time should be spent doing practice problems, reviewing CATs, learning (and practicing) tactics, etc. There's nothing wrong with taking a CAT about a week before your actual Test Date, so you should be able to do so without any negative repercussions.
1) What materials have you been using during your studies?
2) Which practice CATs have you been using?
3) Have you been taking the ENTIRE CAT each time (including the Essay and IR sections)?
GMAT assassins aren't born, they're made,
Rich