Solder will normally stick to copper pads with ease. The reason that your solder doesn't want to stick is probably because of oxidization. Copper pads, solder and even your soldering iron will oxidize over time - quicker when it's hot. Flux helps with this for 2 reasons of note:
1) meeting point in the low 100s means there is fluid present that helps speed up the transfer of heat from the iron to the pad/wire/solder.
2) because it is slightly acidic it removes the oxidization from the pads.
For easiest attaching I suggest you tin both the wire and the pad. Use flux on the pad if you have any before tinning and then more flux when joining the wires to it. The flux really does help.
Edit: also worth pointing out that flux burns away quickly so if you keep the solder liquid too long or melt it many times the flux will be gone and the metal will get harder to work with. If your solder is not shiny or it goes dull when cooled is likely got no flux left. You can fix it by just covering in flux and flowing it again
If you don't have any flux available then you could try a technique I've used to tin pads in problematic locations. I've been told it is similar to something called `tig welding` but I don't know what that is. Basically you hold the unmelted solder on the pad and bring the iron down onto it and then off again quickly. Almost as though you're smashing the solder into the pad. I you have a cone tip in the iron use the side of it so theres larger surface area. Repeat untill your got a little blob of solder on there then join the wire.
Hopefully this helps you out
