I can't get a decent white background on my product shots
Here are few samples
After editing in Photoshop
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The diffused light source is a big problem. Harder edge lights will help create some edges on the bottle and separate it from the background. If you replaced the soft plastic bottle with a glass bottle, you would see the specular highlights that would help define it. With a soft plastic bottle on a soft white background lit with soft diffused light, you are fighting an uphill battle.