I have a brand new DJI mini 2 fly more combo. So, with 2 extra battery's. I ordered a several more products, like an Ipad mini 5, microSD cards, extra long cable, tablet mount, Care-Refresh etcetera.
And I bought a new extra battery, so I have 4 battery's now.
I waited until all was completed.

Yesterday, I loaded the three battery's (from the fly more combo) and te controller. No problem.

After the three battery's were charged, I installed the DJI Fly-app on my new iPad mini.
All good. The app is oke, the connection (the DJI mini 2 and the controller) was paired.
The new firmware was installed. And I registered the Care-Refresh. So, all is good and ready to fly.

As I said above, I have a new fourth battery.
I wanted to charge the new battery in the DJI-charger. I put one of the fully charged battery's in the mini 2 and I put the new empty battery in the charger with the 2 fully charged onther battery's.
In the morning, I saw only 2 lights flickering whole the time. So after 7 hours charging, the new battery did'n charge.

So, today I charged the new (fourth) battery in the DJI charger to load, without the other two battery's, and it went well. So now the fourth battery is now charged.

What did I do wrong?
Why couldnt charge the new fourth battery in the charger with two other battery's?
Can I charge my extra battery only when he is alone in the charger, without the other battery's?

I read something about firmware of the battery's, but couldnt find info about this.
In the manual and on internet I read the DJI charger is an intelligent charger. And I read the charger is charging one battery, then an other and then the third one. And he does.
So why could'n I charge my fourth battery in the same charger with the other two?

Questions, questions...

Strange.
Never happened to me.
It starts with the 'least empty' one that is not around 100%.

You can check the firmware if you power up the drone and connect the controller + device.
After a little while the fly app will state there is an update for the battery (or not)

Hello there AdriA3. I am sorry for the trouble this has caused and thank you for reaching out. As stated above kindly please check if the firmware the said DJI Mini 2 intelligent flight battery is on it latest one and try to charge it once again to see if the issue will persist? In addition kindly please press the function button on the charging hub once to activate charging if charging does not begin automatically. Thank you.

Thanks all!

What I tried is this:
I put a little sticker on my new battery, so I can recognize the new bought battery. They all look the same. hihi  
Then I put a battery in the mini 2 and I paired the mini 2 with the controller. Pairing was good, but no firmware update for the first one. I repeated this battery for battery.
In the first 'round' there was one battery who was updating the firmware of the battery.
I did the same again for all four battery's. In the second round, the flight app updated the firmware of two other battery's. In the third round, the last battery was updating te firmware of the battery.
So, now all four were updated. After this, all the battery's were charging perfect!
Maybe I did this wrong, but it works! Now I can charge all the four battery's in the charger. I thought my new bought battery was broken and I had to return to the shop. But it works now.

It is a pity, DJI don't speak about a battery-firmware update in the manual!
I paired the mini 2 and the controller and updated the firmware. But I didn't know you can update the firmware of the battery also!
And apparently you can update the battery's as well!
Now all works fine!

Thanks all!