Are you absolutely sure that you are providing enough sunlight?
I am wondering if inadequate sunlight is keeping them limp? My grandmother gave me 2 before she passed back in the mid 90's. These were by no means beautiful plants but they had a trunk to hold them upright and several stiff main branches or limbs. She cared nothing about new soil or plant food. She was just an old farm girl from north central Connecticut yet her Christmas cacti were gorgeous! She had them in a corner of her den on a little drop leaf table and they got sun from like 9 in the morning until 3 or 4 in the afternoon and when these things bloomed, it was like a pink waterfall!
I did not move them with me when I went to Florida but gave them to a friend. I wonder if they are still alive but grandmas plants were never droopy, in good sun and were kind of neglected. Yet they bloomed like crazy.
