Hi Crystalview, Christmas cactus are fine outside in the shade not in Direct sunlight. I think it could be soil conditions! Not repotting after a year, or over watering or root rot maybe a problems. This time of year new clades (Leaves) should be coming on plants. Christmas cactus prepare for flower buds in the fall. It's not to late to save plant. I like Black Gold Cactus mix and worm casting.
I add a New Schlumbergera cactus season forum 2018 -2020 to encourage new people to join. I hope this helps you,
