I recently saw a forum post where someone thought something was wrong with their Glock because it would not chamber the round. They shot a video of what was happening and it was simple, they were riding the slide and not letting the springs do their job. There are two ways to rack a slide, slingshot and power stroke. There are a handful of firearms which require a slingshot position, the Ruger Mark I/II/III and 22/45 come to mind, other than those firearms you should always employ a power stroke when possible.
So what’s the difference, slingshot is like pulling a slingshot. It works, but puts your hands in an awkward position if you keep the muzzle on target. Try it, hold your strong hand like a gun and point in front of you downrange. Now take your weak hand and get in a position to pull the imaginary slide back like a slingshot. It’s an awkward position and only becomes comfortable if you rotate your dominant hand (the gun hand) to the side which would be off target. Power stroke is much more comfortable and works for everyone. Slingshot relies on your hand strength to pull back the slide. With power stroke you can use your arm strength to push the slide. Same scenario, strong hand is the firearm, weak hand operates the slide. Approach the firearm with the hand open and grab the back of the slide with the weak hand. The palm of the weak hand is positioned at the thumb of the strong hand and the fingers wrap across the back sight and are position on the side with the back of the strong hand. You can either push the strong hand forward and release the slide with the weak hand, or pull the weak hand back and release the slide. Most people will find it easier to make the weak hand/arm a rock and solidly hold the slide in position and push the strong hand forward. When the strong hand goes forward release the slide with the weak hand.
Now there is yet another way to move the slide forward and that is to use the thumb to push down on the slide stop. This is a bad idea in the long-term. It can wear down the edge of the slide stop and eventually make it difficult for the stop to hold back the slide. I have a firearm where it was continually released using the slide stop and now it will barely hold back the slide and the slightest bump releases the slide. Use it as a slide stop, not a slide release.
The bottom line of it all is to remember to release the slide. Do not ride it back down the frame. Pull it back and release or hold position and push the firearm forward and release the slide. I picked the first YouTube video I found on the topic, viewed it quickly and it demonstrates the slingshot and power stroke if I did a crappy job explaining them. If these concepts are new to you, take a few minutes to view the video.