Heltec Wifi Lora 32 V2 doesn't work

Hi All

About a month ago i ordered 25 of the heltec Wifi Lora 32 V2 (868MHz) boards. Now the problem that occured is that some boards seems not to work i uploaded to each of them the same code to send someting to the gateway (not my own but from a provider that has covverage of whole belgium)
On some Boards (± 15 boards) i receive the message at the gateway every time (each hour) and with some boards(±10 boards) it does not recieve something on the gateway. I already have contact with Aaron lee from heltec to look at this problem and test different things but maybe someone else have this problem and maybe a solution.

Thanks in advanced !