The mAh is not a measure of the power when you pull the trigger. Its the total power it can hold so how long it should last.
I think those batteries are faulty especially if you say other batteries work.
That does seem to be a low battery but very strange as its an 11.1
Your Imax B6 looks quite different to mine. (I have the V2)
There should be a specific battery test mode in the top menu.
Not sure if these need 7.4v or 11.1v (I'd always go for 7.4v to be safe but someone will confirm)
I'd suggest something like:
https://www.vapextech.co.uk/airsoft-7-4v-2600mah-25c-lipo-cranstock-battery-with-case-lp305-vapextech
The Milliampere Hours (mah) determines how much total power a battery holds (ie how long it will last)
The C rating determines how much power it can deliver per trigger pull (20c is the lowest and 30c is usually the highest you can get)
Remember you will need a GOOD intelligent charger to take care of these batteries and prevent fire.
This is the cheapest I can find with a quick search.
https://hobbyking.com/en_us/turnigy-accucell-us-plug-c150-ac-dc-10a-150w-smart-balance-charger.html
When I buy from abroad I always include my UKARA with label in the address to make sure it appears clearly on the box.
Not had a problem with customs but had to pay taxes a couple of times.
Welcome ✋
I also started in my 40's to share activities with my son. Of course he rushes off and I don't see him for the rest of the game except in passing to the respawn 🤣
That's the beauty if this sport. We can all enjoy it with different play styles.
I think us, more mature players, are better at tactical sneaking around with the occasional banzai charge 😁