What could be the problem ?

Seems like you have some stretching there, your plant is looking for more light and stretches. Try lowering your lamp a bit (depends on lamp power but I believe about 10cm lower would do fine). To fix the stretching already done to the plant just fill the pot and cover the stem leaving about 2.5-4cm of it exposed, the covered stem will start growing roots and the plant will keep growing from that point. I also took a quick look at your diary and other than it being a little hot (don't let it go over 30 degrees Celsius) it looks great!! Good luck! and happy growing :)