This problem is unfortunately too common in my field (videogames on mobile devices), and the usual reply from the designers “Peole are so dumb! My design is perfect!” is unfortunately the wrong one. If your user does not intuitively understand your design, then your design is bad. Period. Don’t try to add a tutorial pop-up or anything. Go back to the drawing board, check the 6 design principles above, and come back with a solution that really works.