Nate - how long does it take the bacteria to reach full attenuation? When I try this, I always seem to get a LOT of acidity but little drop in gravity.
Lactic acid is barely lighter than sugar. That's your problem. The SG isn't a good guide for when it's done. If you taste it, and it tastes super sour, then its done. If you taste it, and it tastes sweet, it's not done yet.
That makes sense - you're just producing acid with the bacteria, not alcohol.