Its too short for day 7 ,what is the problems?

Your seedlings look fine. The first 3 weeks always seems to drag on a bit slowly. I would start by leaving the light on for 18 hours a day. A couple of your seedlings are a little stretched which means they are reaching for more light. There is no way to say exactly how low you should have the lights heat can be a big issue with your delicate seedlings. I would definately drop it down to 48" then slowly lower it about 6" every few days to take it down to 36". if the respond well and no signs of wilting take it down to 30" etc.Some growers go closer I've seen some as low as 15-20" but not everyone has the same setup. But wait until they have a larger root system before you try. Always do it gradually when you will be able to watch their reaction. Good luck hope this helps.