This is often seen when the brush is set to Auto Mask because the mask is applied only to pixels that are the same or close to the pixel under the center of the brush, thus not all pixels are masked and "holes" are left. Uncheck Auto, using it only for when you are brushing at the edge of the desired area. Another technique is to not worry about going over the edge and then using Erase with Auto on to remove the excess.
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