phasing ?

The pan law theory doesn't, er, pan out because even if you were using the -6dB center pan law that would not account for a 7.5 dB drop.
 
Completely remove (not just bypass) all effects and see if the problem persists. Although unlikely, make sure there aren't any clip effects. Also verify that the track interleave is set to mono.
 
I am suspicious of the track effects because one or more of them may be forcing interleave toggles, which happens when you mix and match mono and stereo effects. If any of them were to introduce a phase shift, that could reduce the level. However, I'd expect that kind of problem to be audible, as a thin-sounding track. It's also possible that one of them is causing a level drop even when bypassed, an unusual but not unheard-of behavior.
 
 
 

One other that's not visible in your screenshot is the fader level of the Master Buss. Make sure it's at 0dB!

 I FIXED IT!!!....
But very weird.
 
I decided to create another Bus and set this as a New Master Bus.
Wallah!... got my missing gain restored.
So I deleted the original Bus, and renamed my new Bus to "MASTER"
AWESOME.
 
Now another point I would like to make, is that this song was originally created many years ago in Sonar 6, and it is only recently that I decided to re-visit it and use my improved mixing skills to get a more polished mix.
I located the old Sonar 6 Producer Bundle file from 2005, and imported it, (without any issues... apart from a few missing plugins that I don't have at this time), and away it all went.
 
Hmmm... a tricky one eh?!?!?
 
Cheers, and thanks for your inout.
Adrian.

I just hate it when I accidentally "fix" a problem without knowing what it is that I did. That's like scaring away a burglar, only to forever ponder how long it'll be before he returns.
 
 

AdrianNewington
So I deleted the original Bus, and renamed my new Bus to "MASTER"

Has the Vocal Reverb bus been redirected to the new Master? I was going to suggest that the reverb bus signal might might be cancelling some of the direct signal from the track.

The reverb send, including the other sends, and all the FX's were turned off.
Just implementing a new master bus alone Fixed everything.

Doesn't appear anyone ever suggested to check Gain/Volume settings in Offset mode, which would be another likely cuprit. You might re-open the oriignal bundle and check for that.

How do I do that?
I'm not familiar with this offset mode you suggest.
Thanks

Hit "O".