Would I be correct in presuming that the digital readout on that iron is in Celsius? If so, try soldering at 400 - 425C. Or, if it's in Farenheit, try using between 750 and 800F.

Use a broad, flat tip and resist the urge to 'jiggle the wires' until you are absolutely sure that the joint has cooled down to where the solder has solidified.

Just another $.02.

Bob