I know this is going to sound crazy but it has worked consistently to stimulate Brett several times for me. Try adding some pelletized hops. I often brew (sometime award winning) saisons finished with Brett and a little dry hopping does this style a favor. The first time I realized the effect, I had a secondary pitched to Brett for about 2 weeks with no apparent activity. A couple of days after adding the dry hops, fermentation activity was apparent. I've repeated this process at least 5 time with the same results. After adding the hops, final gravity is reached in about 4 weeks. I've done this with Brett pitching gravities from 1.008 to 1.015 to finish from 1.002 to 1.004. For my taste, I use Brett fermentations at 60F, so your temp is fine (more funk, less fruit at higher temps). I normally use 0.5 to 1 ounce of Saaz.
My theory is the pelletized hops have trapped air in the pellets and this stimulates the Brett.