Have you been sure to remove the old pollen-covered stamen before trying to cross-pollinate?
The sticky anther is hidden in this species.
Once the stamen is dried up, it will often fall off by itself, but you may have to remove it with tweezers to expose the anther.

If you've already done that, then I'm afraid I don't know why it's not working.
Maybe there's not enough pollen? I use a soft-bristled paintbrush and wipe pollen grains all over the anther.

This species does grow exceptionally easily from cuttings, however.