I have two 32 year old Christmas cacti I got when i got married. I have taken great care of them i thought. Well 1 of them is doing so nicely getting new leaves and just doing great. I live in southern Ontario so in fall i put them in my west window. Has always done great. Lots and lots of flowers. Well the other one has leaves dropping like crazy. They both have been repotted the same time and have 12″ pots. In spring time i put them outside on the southern side of my house which gets half shade from noon till sundown. I don’t know what is wrong. Two years ago they had 100’s of flowers on them both. I just don’t know could it be that it’s just getting old?
From your description, you have two identical plants that have been treated in exactly the same manner and kept in the same locations for 32 years and now one is not doing well while the other is thriving. That scenario seems improbable, so most likely something has happened to the ailing plant that did not happen to the other one.
Leaf drop on a Christmas Cactus is not a good sign and has many possible causes, including over-potting, improper watering, improper light and temps that are too warm.
I need more information from you as to what is different between the two plants other than their current condition. My biggest concern is the pot size. 12 inches is very large for any Christmas Cactus and substantially increases the possibility of root rot. If you can provide details on the repotting that would be helpful.