soldering wires between two points on a perfboard
That's probably because the wire isn't clean. Use extra solder with flux kernel, the flux cleans the wire (reduces any oxidation). The best thing is tinned wire, even older solder should flow up that wire easily.
Tinned wire is also easy to lay out on the perfboard: just hold it/heat it with your soldering iron, and the solder will flow nicely over the pad and wire.