Question on fluctuating volume

I have an older version of Guitar Tracks Pro and have been creating songs for a few years now.  Every now and then I have noticed on some of my tunes I have a problem and I am not sure what is causing it.  I didn't notice it on my most recent song until I converted the song into an mp3.

The problem is this:

Every now and then, I will notice that the volume of my entire song will fluctuate.  As an example, in my most recent song, the volume of the entire band was reduced considerably once the vocal track started.  I didn't do anything in the song or in the mix to make this happen.  I noticed it happen at a couple of other spots in the song...some of which made no sense at all.

Can anyone give me any hints for what I should look for that might be causing this?  Is there anything built into Guitar Tracks Pro that automatically adjusts the entire volume of all tracks based upon the input of "the loudest track"?

I would appreciate any help on this!