I have rooted my old NOOK HD+ 32Gb and installed a version of Android
7.1 Nougat on it, now I am struggling to achieve the unlock action...

I needed an Android device, to enable me to run an Android only app.

I wonder if anyone is familiar with the unlock action necessary? I am
new to Android.

When locked, it shows a tiny padlock in a circle, at the lower centre
of the screen. Tap the padlock and it suggests 'swipe upwords to
unlock' - I am finding it quite a tricky exercise to get it to then
unlock, usually it takes me several attempts, so I'm wondering if maybe
I'm using the wrong technique. What seems to work most often, is a very
rapid swipe from the bottom to the top of the touchscreen.

Perhaps it's the implementation of Android combined with the NOOK?