Problems with synchronizing sound and voices when compressing a blu-ray movie

When I try to make a compressed BD-25 1080p blu-ray with one audio sound and one legend, from a original blu-ray, it happens that there is a small gap between the movie voice and the sound (sorry for my English).
This only happens when it is necessary to compress the movie.
Can the experts of this technique what possibly am I doing wrong ?

Thanks in advance.

JM