it's just a bad luck,
Imagine you have a wheel like a fortuna wheel that spins if it hits the 40% Fail case you will fail if it hits the 60% you will succed.
This is how game determines if you will get the stone or not. There is any thing else that is taken to the equasion, not the number of times you tried or the nomber of times you failed. With this method you cant get succes every time if you're a bit lucky and you get a good generated number or you can faill 100 out of 100 tryes
Yeah I have reached that conclusion, too. Then Imma' ask the Old Lady 'bout my fortune everytime I wanna refine ores, so I know what to expect xD