Procreate is a great art tool for the iPad. With the release of version 3.2 earlier this month they made some changes to the layers. Most of them were really good, like the ability to select multiple layers at once. One change that has caused some confusion is how Alpha lock works.
Now you have to use two fingers and swipe your layer to the right instead of one finger.
Also instead of white brackets around the layer you nows see a checkerboard pattern behind your layer to show that alphalock is on.
I made a video showing the other changes Procreate has made in their most recent release.