there are little nerves in the skin that no one can see. sometimes you hit one, sometimes you don't. this is not remotely a good reason for questioning your qualifications for nursing. if you are giving the subcutaneous medication properly (and you can check the book for that-- smallest needle, right depth and angle, right drug), it's luck of the draw.
remember that a smaller syringe (such as you would use for a subq med) will push out harder than a bigger one, so if it's the delivery pressure of the drug and not the needle insertion itself that appears to be the problem, inject more slowly.
as an experiment to demonstrate that, take a 2cc syringe of water and a 30 cc syringe of water. push them each as hard as you can, as fast as you can, and see which one hits the ceiling better. higher pressure develops from the smaller plunger. think of how stiletto heels dent a soft floor, but flats on a woman of the same weight do not.