Hello folks,
I did a search for this issue, but found nothing. I apologize if this has been dealt with ad nausem and I just didn't use the right search criteria. I just recently integrated a Saffire Pro 40 into my studio. Upon loading a project previously done with an M-Audio Profire 610 I noticed several anomalies:
 
(1) *Certain* track levels disappeared, although audio still was heard,
(2) I inserted one of the preset BGV effects into the BGV buss and the audio just disappeared. Turn off the effect and the audio reappears.
 
I did close the project and reopen, as I think I've heard that works, but that did not solve the problem. What I did do was create a new buss and route the BGV tracks to it, added the effect, and the audio reappeared.
 
Any ideas? And, no, my mind was not chemically altered :-) I had a witness.

I am not sure what effects you talking about, but may be check Input and Output settings, also make sure you set the driver (ASIO or other).

Thx for your response. The FX I am referring to are from the new Vocal FX Presets. I/O was correct and ASIO was active. Let me be more specific. I installed the Pro 40 two weeks ago and have had zero issues. However, I reopened a particular project yesterday and out of the blue the levels disappeared on the lead vocal channel. However, the audio was there. I checked the meter settings, the buss, and all was well. My solution was to create a new track and move the WAV file over and things were fine again.

On the same project, minutes later, I worked with a buss for background vocals that previously had no FX but was working correctly. I inserted the Background Vocal Focus FX. When playing the track with the FX engaged there was no audio. Turn off the FX and the audio reappeared. I am bewildered. So, I opened a new buss, set the I/O, inserted the effect, and the background vocals miraculously reappeared.

As an owner of a custom business software firm, I know software can go weird - but there is usually a root cause. I don't *think* I did or didn't do anything to create these issues. There is a consistent solution to these anomalies, creating a new track or buss, etc. I wonder if these problems are an indication of file corruption, just general weirdness, or what. I'd like to know if others have experienced similar issues, and hopefully learn why they happened. Maybe there is no explanation, but at least there is a solution.

I've had similar instances where all the tracks are playing but somehow got "cut-off" from the master buss. Usually a restart fixes the problem. Not in your case but these may still be related bugs. Maybe its third party plugins in my case. Hope they find a fix for you.