Dear evoorschot,
this could be because of the wrong “base” color. We always recommend to use a mixed black if you plan to use a hot stamping foil.
Make sure that you are using CMYK/W with 100% for each color.
Also make sure that you press the “base” only for 5-10 seconds with low pressure and the hot stamping foil with highest pressure and for 30 seconds.
