Stellis species leaf tips turning brown what is the cause?

[Roberta]

Stelis do mostly like to be a bit on the shady side. They like to stay damp (but not soggy), and like nearly all Pleurothallids, like good water, and very dilute fertilizer. Those brown leaf tips do look a lot like the result of salts in the medium (fixed with new sphagnum) or too much light or both. But I think you are moving in the right direction with the repot. Clay was also a good choice, since most of them like the cooling that gives the roots.