The bulldog is soooooo far back in a bullmastiff's past that it isn't likely to come out. Most likely the problem is either inbreeding or breeding that isn't true (there is possibly a mutt in there somewhere).
If both parents are registered (AKC or UKC preferably), you can rule out that there is any other breeds in the dogs' heritage. Also, with pedigrees, you can check to see if the parents are possibly from the same lines. (Which I'm sure you checked prior to the breeding.)
If both parents don't have papers (or are registered with a 'basement registry') anything is possible.
But, I'm not an expert by any means, so I cannot tell you for sure.
Either way, I'm sure they will be adorable pets!