When I tried to transfer some endangered plants to their original habitat they failed to survive, although they were transferred during the rainy season. Ideas as to why?
There can be manifold reasons: different conditions at cultivation site and original habitat, the phenological stage of the plant when you did the transplant, issues of competition, pests etc.
The best time to transfer the plants is 15 days before the rainy season, or if you are living in areas where the rains are very less initially, then transplant them after one or two showers, during the dry spell. Do not water them. Kindly transplant with the soil in which you had hardened the plant, so the plant will have the same microhabitat. All the best...
The survival and growth of a seedling is influenced by which plant species was leaved in the site you transfered, even though the site is a habitat for transfered species. This is because of pathogenic fungi or allelochemicals or limited mutualistic fungi by the plant species grown before transfering. So I recommended you survey the relationship between two species.
During cultivation of plants at any particular site/area/conditions, plants get acclimatize to that environment only. The acclimatization process may take time depending upon the plant species you are handling. Similarly, when you are trying to transfer the plants to their natural habitat, initially they may not respond.
Further I want to know that why are you trying this when plants are already endangered. If cultivated plants are endangered, one can protect the same by cultivating them on large scale.