Basil won’t sprout

I planted these 6 days ago. From left to right I have one column of tomato and two columns each of basil, thyme, and marigold. The soil is a seed starting mix. Each seed packet said to plant 1/4” down so that’s what I did. In each cell I made a divot with my pinkie, put in two seeds (maybe four in the case of thyme just because the seeds were too small to handle easily), and lightly covered them with soil.

As you can see, all of the marigold cells have seedlings. Five out of six tomato cells likewise contain sprouts, and nine out of twelve thyme do as well. The basil however has done nothing.

I keep the tray covered, and I believe I keep the soil at a consistent moisture. The house is about 66F, but the shelf is near a heating vent so it’s a couple degrees warmer for the tray. I did check this. Is there anything else I could be doing?

The second pic is just to show how I have it set up.