Can you show us some examples, or perhaps be more descriptive? Since I am not where I would like to be in terms of skills it would be helpful to see an example of the problem.
Make sure the opacity and or flow are not at zero, and be sure and check what blending mode you have it set to. You can change the opacity and flow on accident pretty easily. If, while working with the brush, you press enter, it puts the cursor into the opacity box.