I made this design:
Unfortunately, when printed on apparel it looks like this:
Most of the design is not printed, only very bright places are visible. This design is painted with white color only, so in darker places those white pixels are nearly completely transparent.
The method that is used to print designs on apparel is called Direct-To-Garment and most DTG printers don’t deal well with low opacity pixels.
If you want your design to look good on apparel, you need to adjust it so that most pixels have higher opacity. Threadless notified me about this issue, and this is how they adjusted my design:
Now, when printed on apparel, the design should be much more visible.