I had a lot of trouble to solder the connectors on new HiFiBerry DAC.
I appears that the solderability at the connectors is very bad. Maybe they are too cheap or has been in stock for too long?
Any way, I think you have change the ones you are delivering with the DAC.
HiFi nerds always talk about connectors and pay a lot of money to get the best ones, but what would that help if the bad connections is in the electronics inside the gear:-)
Bad soldering on connectors