I have Affinity Photo 2. I have been having the same issue. I ,think, my issue is this... the app resorts back or is stuck in white color and will not allow me to change colors. So, painting on a white background makes it seem like nothing is working. If I try starting a new sketch file then it works for a while. I can change colors, etc. Then all of a sudden it changes back to white. I can paint in white over a darker color but I can't change the color anymore when the issue happens. I tried just once to change tools and then change back to the Paint Brush and that seems to fix the issue at least for a while.