Heat embossing is a great way to add dimension and extra interest to your handmade cards. While it is important to use the correct ink (a slow drying pigment ink is perfect), it is also important to make sure that you have removed the static from your card. A build up of static will attract the fine embossing powder granules and this could result in unwanted spots of embossing.
In order remove any build up of static before you start work, try rubbing the surface of your card with an anti-static bag. If you don't have an anti-static bag then try dusting the surface of your paper with a dryer sheet. Another DIY anti-static solution is to brush talcum powder over the surface of your card before embossing.