I use Adobe Lightroom for a few years. Every version I used has the same very annoying issue: when I select the Adjustment Brush tool and start painting on the image, sometimes it does not do anything. You know, when it works, then you first see the current brush anchor point immediately and you see the brush effect applied to the image. When it does not, then there is no effect and no anchor point appears. It seems that it happens with some images rather than with all of them in a session. Sometimes, it can be fixed by switching the brush preset to another one, but most of the time there is nothing to be do to fix the behavior.
I believe that the functionality is implemented using some sort of a state machine and the state machine definition has an error. Guys, please fix this bug. I understand that this must be even hard to reproduce, but in the cases like this you need to change the approach and run brute force automated tests (you do have them, right?) with the Cartesian multiplication of scenarios to find the cause or, perhaps, to re-implement the feature from scratch to get it done right. Do whatever it takes to me, your customer, and the other customers like me happier just a little bit.
I am curious-
Reply with a screen-clip of the top panel when you have the Brush tool selected.
It may give a clue to an answer.