A softer rod is less forgiving and demands good casting technique. Softer-action rods will easily create a closed loop if the stroke is not smooth, or if the rod is being overpowered. When you change the stroke's direction prematurely, the line doesn't have time to completely unfurl and this too can create a tailing loop. Not waiting long enough on the back cast is one of the most common errors for both beginners and intermediate flyfishers. As a rule, stiffer rods require shorter strokes and respond to more power.
Softer rods require a longer arc and more finesse.
Most importantly, practice, practice, practice. Cast in your backyard, in the street, during your lunch break. Casting is far from easy to perfect and like a musical instrument it takes regular and diligent practice. The more you cast, the more you will learn, feel, and improve.