In the latest update to Apple's iPad/iPhone operating system (iOS 10), they have changed the behaviour of the lock screen.
You now need to press the home button your device in order to unlock it. There is a way however, to enable the functionality we had before iOS 10, where holding your finger on the home button (without pressing it) will unlock the device as well.
The option is somewhat hidden within the settings. To access it, do the following;
Go to Settings > General > Accessibility > Home Button and enable the option to Rest Finger to Open