I saw Jess from Wyeast give a talk about sour beers. The main points were that lactobacillus usually can't handle high IBUs, alcohol, or low pH, and it likes to be warm. So if you were above 10 IBUs then that doesn't help, although since it was a BCS recipe it's probably fine. It's possible that the yeast got going quickly enough and got the alcohol level too high for the lacto, although that doesn't seem likely for a Berliner weisse. Or if the fermenting yeast combined with the lacto and brett dropped the pH quickly enough, the lacto could be unhappy. You said around 70F, so that is generally warm enough but I think lacto prefers to be even warmer. So at this point I think you should warm it up and see what happens. Take it to 75F and give them more time to work. The pediococcus could still be doing something in there to give you more sourness too.
All of that being said, Ron (the brewer at Cascade Brewing who won Gold and Silver in Sour Beers in 2009) told me that they don't add their lactobacillus until after primary and then just wait for the beer to get sour enough.
I think for the Flanders you should pitch the blend and let it go for a while and see what happens. Patience.