I've had good luck with cuttings of scented geranium—which was a surprise, as I'd not had much success with cuttings before.
What worked for me:
The cuttings were rooted in moist soilless mix, not plain water.
Cuttings treated with rooting hormone rotted and died. Untreated cuttings rooted fine on their own.
Bottom heat definitely helped. I removed the fabric cover from a heating pad for pets and put a thick piece of glass on the heating pad. The surface of the glass stayed at around 84°F.
I've learned that cuttings should contain more than one node, so that at least one will sit above the rooting medium. Otherwise, the new growth will have to emerge from below the soil surface. Not that there's anything wrong with that, but you have to wait longer to see evidence that the cutting is growing. The cuttings with a node above the soil line developed faster.