Due to their stiff recoil spring assemblies, Shield 9mm slides can be very difficult to lock back, especially when new. Unfortunately this is fairly "normal" for these pistols, and I'm not sure the factory can be of much help. The best solution I've found is the Handi-Racker, a piece of molded plastic about 1/2 inch thick with a cutout for your slide. Insert the slide, grab the grip, put the edge of the Handi-Racker on some convenient surface, like a table or work bench, and lean on the grip with all of your weight. It will get the slide back far enough to engage the catch. It has never failed to work for me while I've broken in four new recoil spring assemblies on my Shield 9. (Why four? It's a long story...) Here is the website for the Handi-Racker:
Safest way to rack your pistols Handi-Racker.
You can accomplish the same thing by carefully aligning the top part of the slide (above the barrel) with the edge of a table or bench (one you don't mind dinging up) and pushing down on the grip. However, it is easy for the Shield to slip off the table while you're leaning on it, damaging the table, the Shield, or yourself. To avoid such unpleasantness, I consider the $20 Handi-Racker money well-spent.