I have had my Devils Backbone for more than 6 months and it has thrived in the same location, growing tall and strong. I noticed the new leaves were growing in smaller and smaller and the lower leaves were losing color and falling off. I live in Florida. It gets watered twice a week with our sprinklers and we've had some cooler than normal temperatures in the evening but 65-75 degree days with dry patches with some heavy rain. I had a "Pagoda Plant" planted next to it for about 1-2 months which developed aphids causing me to lose that plant. I made a homemade remedy of original dawn dish soap, oil and water to control the aphids and used on the Devils Backbone as well. It upsets me to lose this and I am not sure what to do. Perhaps you could advise me on the Pagoda plant as well.
From my research this plant does best in a container. It can be outdoors, but then can be moved indoors when temperatures are below 50 degrees F.
Since I do not know your growing zone, it's possible you are in a warm climate.
Outdoor planting can be successful in sunset zones H1, H2, 22, 23 and 24.
Make sure the soil is well draining.
Best location is bright, indirect sunlight, though it can tolerate partial shade.
In mid-spring and mid-summer you can fertilize with a water soluble fertilizer.
Since a Devil's Backbone is a succulent, you should water moderately.
This link has information on Aphid control.