Most likely cause is you are not getting the joint hot enough. Apply
the heat for a longer time. Then chill out (pun intended). Losing
patience is not the best way to accomplish this task.

When I did this yesterday I thought my problem was that I was getting
it too hot and was drying out the soldering paste. I heated the joint
for a really long time yesterday and the results were awful. Then I re-
read my instructions and they said to heat it for 4-5 seconds. I
thought maybe I was drying out the flux and that made it difficult for
the solder to go where it needed to go. I thought it was pretty dang
hot when I tried it again earlier. I don't know how it's possible that
the solder wouldn't stick. It's just blowing my mind. I've never even
watched someone do this, though. I bet i'd spot my mistakes
immediately if I could watch someone who knew what he was doing.