Failure upgrading with DFU

[Ronen]

We are working on our development board contain the nRF52840. We also use Bluetooth Low Energy to load wirelessly the updated code with the DFU and it's working properly.
After several DFU updates, some of our boards cannot be loaded by DFU anymore...
I have already connected them to J-Link and made erase all, I programmed directly the SW, disconnect the J-Link, they run great with our SW, but the specifics of several boards still cannot be connected and upgrade by DFU.
Is any idea of this phenomenon?
What do I have to do since I can make them upgrade with the DFU?

* Important notes: it's not all the boards, only about 10% of them...

[Hung Bui]

Hi Ronen,

We would need to get more debug information to analyse what could be wrong here. 
Could you get some log from the bootloader when it fails  ? 

What exactly happened when you try to do DFU ? 

Which SDK your bootloader based on ? 

If you flash and test the stock bootloader from the SDK would it work ? 

Have you tested if a normal BLE application still works on those board ?