I feel like this with dense stuff that bores me.

I found that annotating (if you have a physical book that you can mark up) can help. because in theory to annotate you need to not just read but actually digest the content.

Another thing that has helped that I’ve learned more recently is that if what you described happens, I take it as an indication that I need a brain break. Go for a quick walk, meditate, noodle around on phone for a bit.

Just a quick breather / reset improves the quality and quantity of my mental absorption.

Good luck & remember you’re not alone in feeling like this !!