Hmm, were you pressing the button to get the 400C boost? With that, first you tin the wire VERY WELL, then you put some solder in the nook of the connector (fill it about half), and then you push the wire into the nook with the iron. That will melt the solder on the wire and then the connector, and it should give you a nice even connection without any over- or underflow.

Once the wire goes in, you remove the iron, hold the wire there for a second or two until the solder solidifies, and you're done. I used 12 AWG but it should be pretty close.

EDIT: I just did it again, make sure you hold both the wire and the XT90 firmly with helping hands or something similar.