Some are just harder to root then others. You should just dig up a runner with some roots attached. They'll grow very easily. You can probably just pull it up by hand, might not even need to dig. You only need a little bit of the roots attached to the stem.

I understand P. incarnata is difficult to root--I don't know whether this is what you are trying. Although Karyn's advice to dig up a runner is easiest, there is a trick to get it to root. Actually this can work on many stubborn cuttings. After about a month, if the cutting has a callus, damage the callus--a bit of scratching with a razor blade for example. Dip it into rooting hormone and put it back. Many things will root in a couple weeks following that procedure. I've never tried it with P. incarnata, but it's worked for me with other plants.