To remove of oxidation, grease from PCB
Any grease or oxidation will make it difficult for the solder to stick. If your PCB is unclean, the best way to clean it is to get some household veinger, pour in a container large enough to place your PCB in. Add half teaspoon of salt and stir it. Add half cup of water. Then place the PCB in this bath solution and stir/shake the container for about 1 minute or so... You should have a extremely shiny and a bit smelly copper. Rinse under running water. Dry and start soldering. This tip was given to me by 9M2CF, Uncle Chow.