Just a quick thought, are you using watercolour paper?
It might be that you need more tooth than the paper you are using if you have been trying a particularly smooth cartridge or multimedia paper.
You were asking about pastel mat. There is some good information on this very depressing (so jealous!) video which unusually is a step by step made of photos. https://www.youtube.com/watch?v=UQ9wo6QNAlc
She does mention light pressure a few times....but then she also takes a knife to it as well.
Wish my art turned out like this if I took a knife to it!
The Albrecht Duerers are a bit sticky so I can see you could easily layer too much too quickly and fill up the paper. Try experimenting with holding the pencil further away from the tip so you can't actually press very hard and see if that gives you more control. Bizarre I know, but I found it helpful.