It doesn't just foam on its own, it only foams with agitation. Put a little in a bottle and shake and it should foam. (Not that it needs to foam to work.)
I can confirm that I've measured a pH of 3 even when it's cloudy. On the other hand, it seems like I need to use 1.5 times the normal dose to get to that pH. The water here isn't very alkaline, yet it seems like I need more Star San than the bottle calls for. Anyone else notice this?