Screenshot not lock screen

I have 3 different ways we can do this!

Option 1: Ask Siri : “Hey Siri, take a screenshot.”

Option 2: Double tap the back of your phone!

If you open Settings and navigate to Accessibility > Touch > Back Tap, and then assign “Screenshot” to a tap option, you can tap the back of your iPhone two or three times to capture a screenshot.

Option 3: Accessibility Settings

Open Settings and navigate to Accessibility > Touch > AssistiveTouch, and then assign “Screenshot” to one of the custom actions. Or, you can open the AssistiveTouch menu and select Device > More > Screenshot to capture a screenshot without buttons at any time. 

I just played with mine a bit, if you make a point of pressing the volume up button slightly before the side button, screenshots work every time..

Of if you still can't get it down, maybe ask Siri to take a screenshot for you?

If you take frequent screenshots, you could also play with Settings > Accessibility > Touch > AssistiveTouch, turn on the switch for Assistive Touch, you'll see a white dot on the screen. What this is doing is basically creating a button for you that you can configure to do various things. It will always be on the screen, so might you might not like it. but you can drag it around and place it where you like.

Go into Customize Top Level Menu. If you just want one function, tap the minus sign until it says 1 icon. Tap the one icon remaining, select Screenshot from the list, tap Done

Now swipe up from the bottom to get back to your Home Screen. You'll see that little dot. tap it, voila, screenshot.

If you chose more than 1 icon in the previous step, it'll pop up some options and you'd have to click twice to get your screenshot