I think it's normal to lose that many by day 5. It doesn't stop it sucking though. I think it's very unusual to have many make blastie. My sister just did a cycle with good eggs, no apparent problems, 10 mature harvested, 8 fertalized, day 3 I think she still had 8 but come day 5 it was 3.
My friend is an embryologist and said of my 9 I would probably get 3 to blastie. Obviously some get all of them and some get none. I just hope for something!
I'm pulling for those other 3 for you though!