Allow yourself to get bored.
Boredom is not a catastrophe. It’s the feeling of your brain searching for the next idea. It’s mental effort taking place.
Don’t always reach for your phone. (Listen, I like a good game of Candy Crush myself, but still). If you’re stuck, sit at your desk and look out of the window and wait. Pretty soon you will be so bored with waiting that you will start writing.