Not working for a few images

Hello!

This plugin is not adding watermark to many images. Is there any specific size (dimensions) that is required? I noticed a few images have same dimensions but to some, watermarks are being added and to some, they aren’t.
Also, please tell that when we change the angle of the watermark, it doesn’t change automatically in previous images. We have to remove the previous watermark and then add this new. Am i right?

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