So if you're using a mouse, the fading (or opacity sensitivity) wont work since it is tied with pen pressure on a tablet. If you do have a tablet and the opacity pressure option isn't working, make sure you check to see if the tablet pressure sensitivity option isn't too high/low and also make sure that the you have the feature enabled in Ps.
Every so often, I'll get a weird error where my brushes seem to be set on TURBO opacity/thickness. Whenever that happens I click my left mouse button and the issue will resolve. Hopefully some of these will help you!