Toenails going thick and a bit yellow?

Over the last few months a couple of my toenails, mainly the big ones, have gone thicker than they used to be and a little yellow. They are not painful, but they look clearly different from how they were, and filing them down only helps for a short while.

I keep my feet clean and dry, so I am genuinely a bit puzzled by it. Has anyone had this and found something that actually helped? I would much rather understand what is going on than just keep buffing them and hoping it sorts itself out. Any pointers would be really appreciated.