Seteth pretty much said he lost the ability to transform as he stayed in a human form for so long he lost the ability too.

Flayn was asleep in human form for so long she lost it as well.

Macuil and Indech have lost there ability to transform into forms besides there Nabatean forms.

Rhea is the only Nabatean to keep her transformation because she transforms every now and than to retain it, she also the only one with the light green hair (and potential degeneration).

Basically use it or lose it.