Whitening strips left white spots on teeth?

This was my first time using whitening strips. I used Harmony Life strips from Amazon. I did as the directions said. I put them on right and they stayed on for 30 minutes. When I took them off I noticed that only certain parts of my teeth became whiter. Parts of the bottom edges of my top teeth and also some parts of the bottom teeth. How do I get an even white color?
Whitening strips left white spots on teeth?
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