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How well did you clean the surfaces to be joined? Without a blooming good scrub from a stainless steel brush the stuff won't flow. Yes there is the need to get the parent metal suitably hot too, but unless it is properly cleaned first, it won't work. Brush like mad BOTH surfaces and then clean with acetone.
Aluminium does take a surprising amount of time to raise to the correct temperature, and there are no visual clues other than the brazing rod starting to flow. So when the stuff starts to melt/flow, it all depends on how well the surfaces have been cleaned because the actual application of the rod material is very fast. I mean FAST! Stuff melts like butter on a hot poker and flows really well with capillary action. But the surfaces must be properly prepared
And do not apply heat directly to the rod! It MUST take all its heat from the parent metal. This is what is triggering the flow reaction. If you apply heat to the rods it won't work.
I too had to learn this one.
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