It's not working for me either. For me it's doing the same thing as adjust layer , where it is affecting the whole layer. Brushing does nothing. Galaxy Note 10.1
Has it ever worked for you? Have you tried to restart the application, does it work then?
Otherwise the first thing you will see once you touch the canvas is a preview. Once you press the OK button you will be able to draw it on the canvas - i.e. once you left the fx settings mode.
I didn't realize that what I was looking at was a preview screen. I haven't spent much time with PS Touch since I have found another app that suits my needs. I just tested some of the FX brushes and they work fine. Thank you.