Yeah, this business of waiting 72 hours to see if something (anything)
happens is a good way to see if...something (or anything) is happening,
but if I were using a liquid yeast and saw literally nothing in 12h, I would
dump a packet of good dry yeast on it. Wort is the perfect food for
microorganisms, and you can bet the traces of them are going to work
and dumping their crap in your beer during that time. Of course you might
be able to get rid of the off flavors by aging and letting the yeast digest the
crap, but why do that when you could be drinking in 10 days or less (with a
keg) or 2 weeks in bottles?
Ask not for whom the beer twangs; it twangs for thee.