Why do students struggle with Projectile Motion?

Having taught this concept more often than I can remember I am not convinced that it is if explained properly, highlighted with experiment (my students build marble launchers), and reinforced with simulations and problem solving.

However there are some big ideas that a teacher needs to be conscious of all the time and ought to reinforce frequently. These are

a.The independence of horizontal and vertical velocity;

b. The consistent use of direction (up is positive and down is negative for example);

c. Understanding that the time in the air depends on the initial vertical velocity.

One of the problems that students encounter is the constant nature of horizontal velocity in the absence of air resistance. This is easily answered with a recourse to Newton I unfortunately in most high school physics courses Dynamics is usually taught after 2-D Kinematics.