I have a 39C that I used for 6 months before I read about this problem. Even though mine was doing it, it did not seem to affect the shave. It would work equally well on both sides, with the blade centered or not.
It seems to be a common complaint among 39C and certain other razors, mostly Merkur, but it happens to other brands as well.
Replacing the razor you have now with another one doesn't guarantee that the problem will be cured.
You can hold the blade in place (by the non-sharp ends) as you tighten down the head.
Don't give up on the 39C because of this ... even with off-centered blades, its my favorite razor, and one of the best you can get among the modern models.
I Came. I Shaved. I Conquered.