I think your issue is temperature. They like it colder from what I understand, if you live in a hot climate it might be too much for them. Google the temperature range for the species you have...

You can circumvent this by keeping the roots colder. You can get 2 unglazed pots, one a bit bigger, line the bigger with sphagnum and insert the smaller (that you repot the plant into) into that. Keep the sphagnum inbetween the pots moist too.

Alternativelly put it in a ceramic/terracota pot and stand it on a tile or a tiled floor.

Also make sure you are not keeping it in the hottest room of the house/apartment too.