My mustard seeds don’t pop

I made one of our pandemic favorites tonight, Slow Cooker Spiced Lentils with Veggies. It’s easy, tasty, and doesn’t heat up the kitchen too much.

The first step on the stovetop calls for heating vegetable oil (or ghee) on medium heat, then putting in black mustard seeds and waiting until they pop, 1–2 minutes.

My seeds don’t pop. I heat the pan on the induction cooktop at slightly over half power, wait until water dances, put in the oil, wait another minute for the oil to heat up. Then I put in the mustard seeds and cover the pan.

Two minutes later, nothing has happened; the oil is visibly hot and shaking, but there was no popping, and the lid of the pan is completely clean.

Could my seeds be stale? Do I need to heat my oil longer, or at a higher setting?

We like the finished product, even with unpopped seeds, but I’d like to do it the right way, at least once.

Any suggestions?