aluminum can be a pain thats no dought, its almost like the 2 pices are opiset magnets. as it heats up to puddle they repell each other rather than runnig togather like steel dose.
dont know if it will help but i found that just as it reaches the melt point and starts to repell hit it with some filler and it will join, dont try to wait it out for them to join into one puddle all by them selves, its just not going to happen also a fresh hit with the SS wire brush just befor you start goes a long way to a good start. lots of waisted scraps ahead for ya but hang in ther you will get it in time.