PBA,
Sure they plateau. But almost all improve substantially from their earliest practice tests until their actual tests. You can’t just take one practice test and judge yourself.
I also think that is especially true with a yeshiva guy whose reading comprehension might be weak from lack of practice. Reading comprehension can be improved a great deal just by reading a lot more and taking practice tests (even without a book teaching you methodology).
yytz,
There do seem to be a lot of people that do worse on the real test than their practice tests. I suspect that many of them are not taking their practice tests under the same conditions that you have in the real test.
Take your practice tests in one sitting with only the same breaks they have on the real deal. Also if you do poorly on the real test, take it again. Many law schools will evaluate you based only on your highest score.