I've been working on two songs, adding whatever is needed, such as reverb to a vocal, etc and then mixing them down. 
First one no problem.  I can add whatever I want and it works. 
Second one, whatever effect I add, such as reverb or whatever does absolutely nothing. Mixdown and add Ozone 5 and nothing. 
 
I was able to get around it by exporting each track in that project.  Creating a new project and importing each track to the new project.  In the new project I can add whatever I want, such as reverb, delay, POD Farm Bass Amp, etc and it all works.
 
?????
 
 

Is the FX button lit in the Mix Module? If so, click it (shortcut E) to enable plug-ins.
Are the FX bins bypassed? Right click anywhere in the FX rack area to bring up the FX Rack context menu.
 

That was the problem.  I have no idea how they got turned off.  I've never encountered this before. 
When I did the recordings, I used the same "template" for both songs.  Recorded the first song, saved it and then deleted the music in each track, then recorded the second song and saved it. 
 
"FM"

fireberd
That was the problem.  I have no idea how they got turned off.  I've never encountered this before. 
When I did the recordings, I used the same "template" for both songs.  Recorded the first song, saved it and then deleted the music in each track, then recorded the second song and saved it. 
 
"FM"

Hi. You might have accidentally hit "E", which is the shortcut for bypassing all plugins.
All the best.