The keys to a good solder joint are the cleanliness, flux and the correct heat. "Solder not sticking" might imply you're melting a blob of solder and dropping it on the joint. Don't do that. Apply heat to the metals to be joined, and feed the solder after they're hot.
Those wires look a little large for the pads you're soldering them to. You might want to consider smaller wire, or splicing a short length of smaller wire to the ends.
Tin your wires before making the attachment. Clean any existing solder from the pads with solder wick and flux. Then make your connection with enough heat the the solder flows within a fraction of a second, taking care not to use too much heat.
Thanks! I finally did it and it works well.