Nige52's tip on adding a little extra flux is a good one. I do that sometimes, as it does help the joint wet out nicely.
A 15 watt iron is only good for soldering electronic components. I find a good 25 watt iron will just about work, but 50 watts will do just about anything model related, unless you're making something very big.
I tend to use a set of those 'helping hands' when soldering the deans connectors. I pre-tin the wire, and the connector, then just sweat the two together, perfect result every time, unless you forget to put the heatshrink on to the cable first, not that I've ever done that you understand.....!