Hi rosetea,
When these types of score drops occur, the two likely "causes" involve either something that was unrealistic during practice or something that was surprising (or not accounted for) on Test Day.
If you can answer a few questions, then we should be able to figure this out:
When you took your CATs:
1) Did you take the ENTIRE CAT (including the Essay and IR sections)?
2) Did you take them at home?
3) Did you take them at the same time of day as your Official GMAT?
4) Did you ever do ANYTHING during your CATs that you couldn't do on Test Day (pause the CAT, skip sections, take longer breaks, etc.)?
5) Did you ever take a CAT more than once?
From what you've described, you've used a 'book-heavy' study approach. Many Test Takers who use that type of approach end up getting 'stuck' at a particular scoring level, and it's likely that is what's happened to you as well. To hit your score goals, you'll likely need to invest in some new non-book materials and learn/practice some new tactics.
GMAT assassins aren't born, they're made,
Rich