Grades get you in the door, but they definitely do not guarantee you a job.
Your situation sounds like what I went through last year - I assumed that my grades would automatically land me a big law summer position - and while they fortunately did secure me such a position - I received several rejection letters beforehand.
Your lack of success thus far can likely be attributed to (1) lack of job experience or (2) lack of interview experience. To prove this point, there was a guy in my class who had good, but not great, grades - yet he received like 5 offers from large law firms our 1L year. He had had a great job before law school and knew how to handle himself in his interviews. I was being passed over despite the fact that my grades were significantly higher than his - because I had virutally no work experience and had never been in a professional interview setting.
I evaluated my performance in the middle of the 1L recruiting season, and realized that I wasn't as calm or confident as I needed to be in my interviews. You have to realize that most everyone who has interviews with the same firms have the grades, so now they're looking for personality. I'd suggest looking over the interview section of Guerilla Tactics for Getting the Legal Job of Your Dreams. It provides a good framework on how to approach interviews. After I read the book and used her suggestions, I got an offer from the next firm I interviewed with.
Good luck.