3 Reasons why your mask isn't masking: 1. If you have many shapes, Illustrator won't know which one to use as a mask, so it randomly picks one (bad). Select all the shapes and do CMD 8 (Make compound path) 2. You might be trying to 'paste into' like in InDesign. With Illustrator you need to just select both shapes (one should be on top of the other) and do CMD 7. 3. The wrong shape might be on top. If that's the case, move it to the bottom.
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