Hey
@Quangro!
It could be overwatering. But after checking your diary I saw that you are giving them just 0.6L of water per day, wich sounds good to me. Just be sure the soil is dry enough after giving them more water. But I still don't think is overwatering in your case.
It could be boron, but is a very rare carency. So...
1.- I should say that's a potasium carency. Just give them a little more flowering stage fertilizer.
2.-Less probable but, just in case... It could be light carency. I saw that your COB is 40cm from the plant, try lowering it to 30cm for 2-3 days and check it again.
The good thing about all of this is that you can combine both solutions and just wait 2-3 days.
Good luck! She looks very good to me!