Need help soldering

[speed demon]

I bought a new motor a few weeks ago and for the life of me I can't get the leads of the motor to solder. I want to do a simple butt solder to deans wire but the solder won't flow into the motor wire. It will flow great into the deans wire but thats it. I'm using flux core wire along with ample flux paste and still nothing. I also bought some 4mm bullit connectors but every time I put the wire in the socket filled with solder I can pull it right off with some force after it cools. What am I doing wrong? Is there some prep work on the wire I don't know about? The motor is a Himax and is just a backup but I still want it done right. Thanks.