I make popcorn with butter or olive oil, but still when I add salt, especially kosher salt or coarse sea salt, it won't stick. Is there anything I can do to get it to adhere better?
The key is to have the finest possible salt. One way to do this is to take any salt you like, and grind it fine in a mortar and pestle. This just takes a few seconds, and then you can customize to a particular sea salt you enjoy and avoid the need to buy a special popcorn salt. A rotating action in the mortar and pestle (as opposed to pounding) is most efficient for grinding salt.
Grind the salt in a small spice grinder or food processor until it is a powder.