I use to TMPGenc and DVD lab to demultiplex my files, but sometimes the demultiplexer puts out audio and video fiels that end a minute or two earlier than the origional. its aggrevating. how do i stop that from happening?

Also, dvdlab has a feature that automatically reencodes .mpa files to proper frequency, which is very useful, but for a certain file, it stops about about 15 seconds, give me a 27 second audio file, and wont do the rest..since i use this type of audio file for all of the other movies in the dvd, i want to keep it unform, but it just wont do it, it stops early. why does this happen, and what else can i use to demultiplex to a DVD compatible .mpa?