FAQ

Your not getting enough heat into the wire. Basically the miniscus of the solder prevents it flowing which is what flux is for which is usually incorporated into the solder, though also available separately. Once this thinner fluid melts, it provides a path for the solder to flow onto the surface you are trying to solder.

I guess the 1st step is to ensure your solder has flux in it. Most does these days, but there is a chance it doesn't especially if you are using plumbing solder.

Keep the iron on the wire for a nice long time to get the heat in, then apply the solder. It helps to have a small amount of solder on the tip of the iron as this increases your surface contact area and aids heat flow. Once the wire is up to temperature, apply the solder and it should "get sucked in" and make a good connection. Keep the iron in place for a couple of seconds to allow the solder to fully flow in the join, and after removing the iron, keep everything still for about 10 seconds or until the solder goes solid.

It can help to feed the solder into the join from the opposite side to where you are holding the iron. This has the advantage of ensuring that the entire join is up to temperature and will accept the solder, and also prevents you simply feeding miles of solder onto the tip of your iron, and watching it drip onto the carpet.

It is a nack, but once you have it you will wonder what the problem was.