Ah, I remember when I first learned artificial harmonics and they only came out a fraction of the time. *sighs nostalgically*
If you're getting them, but just not all the time, the best advice I feel anyone can give is to just keep working on it. You'll get used to the different way you have to dig in at different places on the neck and on different strings over time.
Speaking of which, you'll have an easier time getting them on lower strings and on lower frets. Also, cranking up your gain also helps them ring out. As you get better, you'll figure out how to get them all over the neck (again, you have to dig slightly differently on different strings and spots, but that will be second nature after awhile).