Weird problem with batteries

So, when I finished my electric skateboard, about a month ago, I had fun just riding up and down my street, but when I went to school with it, after 3~ km it suddenly slowed down, and when I pulled the trigger all the way I was barely moving, so I stopped, and checked the voltages on the battery, (two 3s 8000mah in series) and one of the battery’s cells was 2.70V while the others were 3.60V-3.70V and I don’t know much about electronics but I do know that that’s not supposed to happen, anyway, had to carry the board all the way to school and back, when I got home I looked it up on google and people said the problem is the factory’s fault, so I messaged hobbyking with all the info and pictures and they sent a new one, one month later here we are, after I connected the new battery with the old one that was working, everything was fine, but today it happened again, but with the old battery that was ok last time, so I’m thinking, mabye its a problem with the ESC? or motor? I have no idea whats causing this and its really frustrating, any info that can help just ask, thanks in advance. ESC:

Motor: Batteries: Edit: just checked if discharging with charger will discharge only one cell, and it discharged all evenly, so I’m even more confused, if only the skateboard discharges it with one cell, then there’s a connection problem?

Bad connection or bad cell

So what do I do? do you want me to take pictures of the connections between the batteries ESC and motor?

Pictures of everything, circle the cells that are misbehaving

Ok, thanks, working on it.

Same thing happened to me. Same esc just I have the 5000 mah batts. One cell went down to 2.9v. Luckily I charged it back up and it has been fine since. I think it was that the cell is weaker amd possibly sags more than the others because I had been pushing them pretty hard. Hopefully it doesn’t happen again.

Do you have a cell checked attached to monitor the cells?

Ok, balance charging all the way up now, I have a small thing that I connect to the balance cable: and I connect it when I need to check the cells while not at home with my charger.

Im having problems connecting my phone to my pc and i have a lot of exams so i dont have time to upload the pictures, thanks anyway gonna try to balance charge all the way and see if it happens again