When you say it "isn't working," please be more explicit and tell us what the problem is. What exactly are you doing, what is the result you're seeing, are there any error messages, and also if you add a screen shot it will help.
My Brush Masking tool isn't working. I work with all other Masking tools and they work fine, means there is no operating mistake
Of course an operation mistake is not only possible, but likely. If there was a bug that prevented the brush from working, then you would not be the first one to be bitten by it. There would be lots of complaints already. Check your brush settings, such as the flow and the density. If one of these two is set very low, then the brush won't seem to work.