My son in law has a Shield 40 and it was the first gun I noticed could be hard to reassemble if when you pull back in the slide it is not lines up with the rails properly. In other words if you are pushing left or right or twisting the slide even a little you can get the trailing edge if the slide caught on the rail and simply ensuring that the slide is lines up properly with the rails makes it much easier.
On some of my other firearms if I try to cause this I may be able to get it to happen but generally doesn't happen easily or often as there is either less play or the gap in the rail isn't at a point where the slide is likely to be able to catch during reassembly.