Without sharing the channel, how can we verify or it will be very difficult to assist. AV sync is a problem with many services.
There are several reasons that AV sync can be off.
If a channel's av is out of sync the moment the mux is first tuned, the channel is being transmitted that way. Broadcasters often resync the AV after passing through the video and audio switchers and mixers. Your TV or advanced AV processors often can be set with different AV delays to adjust timing.
Often DVB channels do not properly reference the PCR (equivalent to sprocket holes in film that provides sync to the display) and timing drifts.when a channel is tuned for longer periods (1/2 day, 1 or 2 days, etc.). Changing the channel to another transponder then retuning or power cycling the receiver will force an AV resync. If the service's PCR PID is listed as 8191 or 8192, the channel does not reference the PCR, which provides AV sync.