Could you post an image of one of the lamps removed from the fitting please?
If there are any markings those would be useful too.
Are they LED, halogen, something else?
We have the Seplos BMS (well known for its chronic balancing system) along with the optional flying-capacitor balancer.
It's definitely worth sanding the cell terminals and bus-bars before assembling and the zinc-loaded paste can't hurt.
What went wrong?? (please don't name your previous contactor)
If you don't score here in a couple of days, PM me and we'll move to the HH dedicated forum.
Photos of the existing works may be useful.
I've had a couple of those, all were fixed by the lovelies in the Kerry office.
I have no idea what they did (there was lots of typing and discussions) but I went away with a receipt and the items were logged as returned by Lazada.
We had one minor bump in the road in that I had to return to the bank to get online banking working, 10 minutes (and more photocopies) and we're good to go.
If you are worried about duty / tax then ask if your seller can ship "DDP" (Delivery Duty Paid) which means that there's nothing to pay at this end, everything is pre-cleared by Thai customs.
Where in Thailand are you located?
Your local university may well have testing facilities, or if not, may be able to point you in the direction of a reputable lab.