I think you already have a great grapse of when and where you should place the line. I especially like the bottom left where there is only detail and shading displayed on areas of importance such as the eyes, cheeksm abd ears.
I believe where you fall short is how you implement these techniques. The lines are very thin, scratchy and zig-zaggy. If you practice you lines to become more confident the will look better as well as the flow of the line. Being able to create a confident and flowly line will make you're art better. Having varied thickness in your lines will make you piece look less flat aswell.
Confident vs Scratchy
Learning and experimenting with different shading techniques will improve your piece .and your own unique style.
Your proportions are off sometimes suchs the the far right on the first image. If you start of with a circle as a base and measure off that with reference. I recommend looking at Andrew Loomis's head technique and Bridgemans head aswell to learn to draw a face accurately. Learning Anatomy is also essential, if you learn to draw the skill you will gain a better understanding of the human face.
Head Proportions
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