Solder that won't stick
When solder loses its flux it oxidizes which makes it very difficult to stick. You can add additional flux to counteract this. Additional problems could be, 


Lack of heat. You will need to get the pad on the pcb considerably hot. You can aid this by adding a little extra solder to the pad to improve thermal conductivity.
If using a soldering iron make sure the tip is "tinned" and shiny. Solder gets duller when it loses its flux. Same goes for the contact and pad you are working with.
Lead free solder, which is used on most commercial PCBs is harder to use because it has a higher reflow temperature. It requires more time or a higher temperature. 


Also note that if the pad you are working with is connected to a GND or power plane it will take longer to heat up.