When I run into this same issue I will press the item in my heatpress for a few seconds in order to push down all of the fibers before I print on it. Once the image is printed and cured, the fibers are permanently cured flat so you won't end up with the little "holes" in your image.
This is a result of the manufacturing process of the item that you are printing on and not the printer itself. It seems like everyone runs into this same situation at least every now and then regardless of the printer that they own, so...you're not alone
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