Hoyas can be grown from a single leaf. But you must include, a tiny bit of the stem.  A nip with your fingernail is sugfficient. This will enable it to develop into a plant. While some plants can be grown from a segment or a single leaf, other plants just don't do anything.

Senecio are very difficult to grow leaves into plants.  The photograph are blind String of Dolphins. If you look at the individual, very plump pieces, these also are blind, they probably will never grow into plants.

All these examples are what is referred to as Blind Plants.

They may grow roots and look healthy, but they just don't develop further.  Or if they do, they take so long.

Another way that some varieties of plants can become blind, is by how you prune them. Hebes, especially those cute little stubby garden varieties, are prone to this. It is always best to cut above a healthy node, that is already showing signs of producing off shoots.

With these, if you cut at the wrong point, that stem may never grow further. It will be blind. 

Cuttings from hebes, also are susceptible to going blind.  Hebes really benefit from just pinching the growth tips out, (nip those pointy bits off in the 2nd photo) thus encouraging the stem to produce new offshoots.  Then prune them down when you see all this nice new growth.  Take cuttings from plants that are show laterals are forming.  

I do not propose to understand it all. These are from my own observations I had with the Senecio plants.  There are other varieties of plants that also grow blind from leaf cuttings.

I was taught about hebe cuttings and blind plants while being trained and working for a Wholesale Plant Nursery.    This does not mean every hebe will go blind, some are more prone to it that other varierties.  The Hebe photos are borrowed from the University of Auckland.