While I have never processed cell phone boards, I wouldn't expect much gold from 18 phones.
Did you remove all the components from the boards first, and just process the boards? Or did you put the boards in with all the components still on them?
Did you test your solution with stannous chloride before you tried to drop the gold? What was the result?
How did you drop the "gold dust"?
What did the "gold dust" look like before you tried to melt it? How much was there?
Maybe others can give some kind of idea on what to expect.
Dave