Plate screening for bacterial cellulase production

I have been trying to screen bacteria isolated from termite gut for cellulase production using CMC agar. The isolates grew on the agar on initial isolation but when I tried to screen them for cellulase production on CMC agar using congo red staining, congo red agar and Gram's iodine staining, I could not detect any clearance zone in the agar around the organisms .

Is it possible for a non-cellulolytic bacterium to grow on CMC agar? What my observations indicate now is that the isolates are not cellulolytic but I am confused because they show good growth on the medium. Please note that CMC is the only carbon source in the medium. I need opinions and advice on this.