why mixer brush tool is not working correctly in photoshop, & full square should be filled with tex


Picture 3

Picture 4
why mixer brush tool is not working correctly in photoshop, & full square should be filled with texture compeltely.
I created a circle with beautiful and colorful gradient because I want to create 3d brush like picture 3. However, my mixer brush tool is not working correctly in photoshop. I clicked alt to absorb my circle with beautiful and colorful gradient, and use mixer brush to draw the 3d brush, but the result turns out to be something in picture 4. It is not something that I want.
Please help !! thank you so much!!!!!!!!!!!!!!
