Answer: Based on your description I am not sure what the problem is, but perhaps this will help you in your trouble shooting. Keep in mind that it is fairly easy to root tip cuttings of this plant so if it appears there is a major problem you might try that as insurance, just in case the worst should happen.
Cactus can be infested with mites and aphids in addition to scale, so check carefully for those problems as well. It is possible that a chemical treatment (such as one used to control scale) could have damaged the plant, if the chemical was not listed for use on this plant.
It is also possible that the plant has been either too hot or too cold, overwatered or even underwatered. This plant also likes ample humidity and can sunburn, despite its name of cactus.
The Epiphyllum species do best in a temperature range between 45 and 70 ?F but will tolerate warmer temperatures if there is ample humidity and they are shaded. The plant should only be fertilized during the growing season from spring to fall, with none at all in December and January -- and it should experience a "winter" of temperatures in the 40 to 50 degree range. Finally, it blooms best when potbound, so do not be tempted to uppot too often. Take care not to overwater after repotting, too.
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