A classic reason solder won't stick to something is because you're not getting it hot enough. My interns come to me with this problem all the time. Make sure the tip of the iron is nice and shiny. Touch some solder on it, and it should melt almost instantly.

Why is soldering not working?

Heat is the number one reason that solder flows. Without it, you'll unsuccessfully try to solder a piece for a very long time. ... Just enough to hold it, but not to pull a lot of heat from it. If it's still not working, consider a charcoal block which reflects heat, making it easier to solder.

What does solder not stick to?

Solder simply will not adhere to dirty or oxidized metal surfaces. Clean any flat surfaces which are to be soldered with steel wool, a file, emery cloth, etc. It's important to take time to clean the surface thoroughly. Scrape any wire to be soldered with the back of a knife or any flat piece of metal (Fig.