Thanks Ian, I worked it out algebraically to be a number divisible by 9, but couldn't sort out the smoke & mirrors on the webpage, until I noticed the symbols were identical for the series of numbers divisible by 9, dohhhh.
Each time you "play" it, the software shuffles the symbols to confuse things a bit, but it always puts identical symbols on the numbers divisible by 9.
I'm a little slow today.......