Hi Benny! I have some good news and some bad news.
The good news is that you haven’t done anything wrong, or failed to do something!
The bad news is that’s because it’s completely out of your control.
Distinct Spells and the distinct Spells achievement are based on the different Spells the game makes you cast. You know how you can’t choose what spell you cast in a Foundable encounter or in combat? It’s that - it’s predetermined based on what confoundable/enemy you run into. So every time you find an enemy that makes you cast a different spell (ebublio, arresto momentum, meteolojinx recanto, etc etc etc) that counter goes up by 1. You’ve probably cast all of the ‘regular’ spells, ie spells that you’re likely to run into often. Now it’s just a question of waiting until you find rare confoundable encounters that require specific spells. I know Dementors are relatively rare, and as far as I know that’s the only encounter that uses Expecto Patronum.
Hope this helps!