You have an iPhone that supports Live Photo (iPhone 6s and later, excluding SE) yet it won't play on the lock screen when you press on it?
When you move or scale it, iOS will disable the Live Photo as you can see above. When it gets disabled like that, tap on it once to re-enable it.
When you tap on it, it should say 'Live Photo: On'.
Still not working?
Make sure 3D Touch is enabled if you have iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus, iPhone X, iPhone XS, and iPhone XS Max.
Go to Settings, Accessibility, Touch, 3D Touch and turn it on.
Also Low Power Mode can disable 3D Touch.
Enjoy your new lock screen! Happy converting with VideoToLive!