A leaf cutting with no growth point

What's happening

Those adorable single-leaf Hoya kerrii are leaf cuttings taken without a node — the small bud at a leaf's base that all new stems and leaves sprout from. Without that node, the leaf can grow roots and stay plump for ages, but it has no growth point, so it almost never produces a vine. It isn't dying; it's simply stuck as a novelty.

How to fix it

There's nothing to fix and no care trick that forces a vine — accept it as the long-lived curiosity it is, or replace it with a multi-leaf, vining Hoya kerrii cutting that includes a node if you want one that grows. To keep the single leaf happy for the long haul, give it bright indirect light and water very sparingly, only when the soil is completely dry, since a lone leaf rots easily.

This is general guidance based on common symptoms; individual plants vary.