Hi there! Because the forum you originally posted in is for beginners trying to learn the basics of Photoshop, I moved your question to the Photoshop General Discussion forum, where you'll get more specialized help.
To help others help you, please read through this article and provide any additional relevant details.
Could you please post a screenshot with the pertinent Panels visible?
What are the Brush settings (in the Brush Panel)?
What Color Mode and Color Space is the image?