what is problem? Is it due to temperature changes or irrigation? The temperatures can fluctuate by 3-4 degrees during the day; could this be the reason?

Its different for everyone. You should water by what the soil needs not an amount. Slowly water to not compact the soil. Dont water all at once, but give a few minutes between adding water so that it can soak it all up and not just run off any hydrophobic parts of the soil creating dry pockets. Fully saturate the pot but only till a very little bit comes out the bottom. Once you've done this, feel how heavy the pot is. Feel it before you water. When the pot feels light, repeat the process. If you keep track of how much water you have to give each time, it will give you the amount you are looking for, but amounts dont really matter. Keeping the soil moist and then allowing dry back is what really matters. As the plant gets bigger it will drink more so your number will change. When it goes to flower it will drink even more and your number will change. Best to figure it out for yourself then throw a set amount of water at it Just my thoughts! Best of Luck, Happy Growing!