Are you direct sowing in the ground? The soil temperatures ought to be in the right range now, as kohlrabi will germinate in soil that is anywhere from about 40 degrees to 85-90 degrees. So, if you're having trouble with direct sowing, my best guess is that maybe it has been too dry and the seeds aren't getting enough moisture to germinate. To get around that, you can soak the seeds in a cup of water for a couple of hours and then sow them directly in the ground. Or, for more of a sure thing, soak the seeds in a cup of water for a couple of hours, then wrap up the seeds in either a coffee filter or a moist paper towel, put them in a ziplock bag and set them on a table or counter out of direct sunlight. Check every morning and every evening to see if the seeds are starting to sprout. Once they start to sprout, carefully remove them from the paper towel or coffee filter and sow them in the ground. Keep an eye on the young seedlings as they emerge from the ground. There's still plenty of pests around that might be eating yours as soon as they sprout (if they have been sprouting at all).

ok, love it! it's a plan! (oh, and yes i was direct sowing)