i see, usually for me when come to area where the lead dont flow nicely and melt in peace, i will use a penknife to scratch the surface before soldering. Even on the selector switch, the small area can be scratched using the tip of penknife, just be careful. Make the surface of contact rough and the solder lead shud flow.
to make it easier, you can solder a layer of lead on the selector switch first, before connecting the pickup wire. When you wanna attach the wire, just twist the end to make it neat and tidy, get the soldering iron with the tip at the edge of the selector switch point, the lead shud melt and you can insert the pickup wire end into it. The whole duration shud not take up too much of seconds.
another area which i make it rough is the back of the pot when you need to connect the ground.
and if still cannot, check the lead that you are using, the best wud be those with rosin core, 60/40 type. Imho, this one can be gotten easily and work best. Without using those solder flux.