Like the Photoshop Clone Stamp Tool, the AE Clone Stamp Tool works best with small strokes. It's useful to make several samples near the blemish. If your blemish lasts for longer than a frame, or if the area changes color as time progresses, make sure Lock Source Time in the Paint Palette is off.
Lock Source Time will apply your cloned pixels from a single frame to all successive frames
By unlocking source time, the pixels will be constantly sampled.
Before cloning, decide which method you want because you can't change this after the fact.
Aligned cloning will keep your sample point even if you start a new cloning stroke.
Unchecking the Aligned option will mean that with every new stroke, you will clone the same sample (until you Option-click again to select a new sample).