Make sure the flow is pushed up to maximum.
Thanks very much for your answer! And which setting do you use for density?
See this link
Also see this link.Pete explains it:
Think of Flow as the setting of the nozzle on a water hose. 100% is going to give you a full blast of water in one shot… where as a lower setting will cause it to come out more slowly. Density determines the strength of the effect that you are applying with the brush. It sets the limit of how much of the effect is applied. For instance if you are applying a lightening of the exposure by 4 stops and you have the Density at 75, that would mean that you are only applying the effect at 75% strength.