Failing to program a third party NRF52832 board with nrf52DK through SWD connector after first time
One application note that I found (just looking for generic SWD information) contains the following note:


  Note
  
  Keep in mind that if the debug pins are disabled, a debugger will no longer be able to connect to the EFM32. During development, it is therefore a good idea to make sure there is a delay between reset and disabling the pins (typically a few seconds). In that way the debugger has time to connect and halt the CPU before the pins are disabled. If you have disabled the debug pins and are unable to connect to your device, see Section 4.3 (p. 13) for instructions on how to perform the debug unlock sequence.


It sounds like you might have failed to do this in your code.