Thicker solder harder to melt?
I used a 25 W soldering iron for some time before I got a 80 W temperature controlled one and one thing I've noticed that helps is to have a bit of solder on the tip of the iron. 

The story told to beginners is not to try to transfer solder from hot iron to the joint and to instead apply solder directly from the wire. I won't say that that is incorrect, but it often helps to have a bit of solder on the tip. That solder will improve thermal connection between the tip, device and the pad. The amount of solder should be just enough so that once the tip is in contact with the device and the pad, solder from the tip is in contact with all 3. At one point you'll see that the solder from the tip is starting to flow into the joint. That is the moment when you should add solder from the wire to the joint. If you've done everything correctly, it will easily melt and flow into the joint and the flux will have the chance to clean the joint this way.