Screen capture isn’t working

First, here is the documentation that describes how to trigger the screenshot:

Take a screenshot on your iPad – Apple Support

Assuming that your iPad has a Home button - to trigger a screenshot, you simultaneously press both the “top” (Sleep/Wake) button and the Home Button. For iPad devices without a Home Button, the process is the same - but instead uses the “up” Volume button. If the timing isn’t right, something other than a screenshot is acheived.

So, to break this down to reliable sequence that consistently works...

1) Press-and-hold the Top Button

2) Press-and-release the Home Button | Up Volume Button

3) Release the Top Button

Now, press the buttons in the stated sequence - quickly; the screenshot is triggered at (2). After a little practice, the sequence becomes second-nature - and you will trigger a reliable screenshot every time.

If you have an Apple Pencil, a further option exists. Using the Pencil, from the bottom-left or bottom-right corner of the screen, swipe toward the centre of the screen; this triggers a screenshot.

I hope this insight and clarification proves to be helpful.