I have sown two different kinds of plants (flower and vegetable) in a small greenhouse, for about 1.5 weeks ago. Now 3 (left up, right up, left down) are starting to emerge, but only on the corners and still not in the middle. Strange, isn't it?
Aha, that could be the explanation! It could have to do with moist. I don't moisture the soil daily, and round the edges is indeed more moist. Above the middle is also an air vent. Great answer. Thanks!