Advertising/connection problems after successful DFU

[Jefferson]

I have some customers seeing an issue with advertising/connection after successful DFU.  Everything was working before DFU.  The changes in the new firmware have nothing to do with BLE, it is only some application layer data changes.  After DFU, it is very difficult to discover the advertisement using different phones both Android/iOS and when an advertisement is detected, the connection attempt will fail.  Only a few cases of this problem can be seen out of couple hundred units.  What could be causing this problem?  I am using SDK 15.3 and SD 6.1.1.

[Kenneth]

One thing that come to mind is if you have some serial activity (e.g. spi, uart, twi) at the time the BLE is advertising? This could potentially (depending on the layout) have an effect.

The other thing that come to mind is if you have some bonding data in flash, and you based for instance on this bonding data may be doing directed advertisement and/or enable whitelist. 

I think if you have access to a specific device that exhibit the symptoms you should be able to narrow it down by some try and fail.