Browser random operation

 
Hello,
 
When i open the browser, and click on an audio loop to preview it, sometimes it works (i hear it), and sometimes it doesn't work (i don't hear it).
 
It works randomly (when i do it again, with the same loops list, it is not the same loops that work - but it is unpredictable).
 
I have selected master bus as the "audio preview bus" (X2a).
 
Thanks
 
Th