Could you maybe post a link to one of the affected products so we could go into the design tool and take a look at what you're doing? That image is so tiny that it's impossible to tell what we're looking at.
Off the top of my head though, I would guess that the problem might have to do with the way you're removing the white from your image - if you're using Zazzle's remove white tool in the design tool, it might not remove it if the color is slightly off-white. In a situation where I want part of an image to be transparent, I usually edit the image to remove the white myself and then save it as a png because it gives you much more control over the process.
My other guess would be that it could have something to do with vivid white underbase printing vs. classic no underbase printing on the t-shirt - but if you're seeing the same problem with candles, that sorta argues against that explanation.
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Cat @ ZB Designs
