Welcome! I can answer your questions very quickly.

For one, you say sprouts, so I'm guessing you either just plant the seed, or germinate it first, then plant, right? If so, the first thing is, seed do not need light to sprout. They will sprout either in the light, or in the dark.

Second, you say you water when the soil is dry. That's your problem right there. Seeds NEED lots of moisture to sprout, and letting your soil dry up will kill it in no time.

So turn off the light next time, and place the freshly planted seeds in your soil, and keep the soil moist. Do not let the soil dry out, but don't let the soil be mucky either, just moist.

Other than that, good luck!