There's a reason they use propane or MAPP gas torches when soldering copper plumbing pipes. All of the above comments about the heat being conducted away are exactly correct. And your bus bar has far more cross sectional area, and is thus a better heat sink, than copper pipe.
When soldering copper pipe with a torch, I usually protect the flow of heat away from the joint with wet rags. Granted in that case I'm trying to prevent the heat from spreading to and damaging sensitive and expensive components like ball valves, and what's really happening is that the water in the rags converts to steam which carries away the heat. But thinking about it, wet rags might also create a region of colder temperature and the resulting gradient might prevent heat from conducting away so easily from your work area.
No matter what, though, you're going to need much more heat. And by that, I mean you need a HOTTER heat source. Equal area under the curve is not equivalent here; slowly raising the temperature with an insufficient heat source just allows the flux to evaporate, and as you've discovered solder doesn't flow well onto unfluxed bare copper. You need to properly prepare the surfaces, then quickly raise them to the necessary temperature to flow the solder so the flux is still "acting" when the solder begins to melt.
I'd treat this exactly like soldering copper pipe: Sand or otherwise mechanically clean the surfaces, apply lots of flux, apply intense heat for a short time, flow the solder, and allow the joint to cool.