I am using Nuendo2.2 for some audio editing and have faced this problem

I have made some music files in ac3 format for which I used vegas6 and burnt them on dvd to play on stand-alone player. They play well but I found that the surround volume is very low. Now since vegas can not edit ac3 file and I had deleted the source mono files I tried to open in Nuendo with channel split and used volume gain for surround channels. They played well as far as they were played through Nuendo itself but when I re-exported to ac3 the file was created. It seems ok but there is no volume at all in any channels. There is no volume heard even after turning the volume control to its highest.

What could be wrong?