Can Radiesse work if Sculptra has failed? I have very severe facial lipoatrophy.

There would be no issue using Radiesse in an area previously treated with Sculptra, but I find it surprising that Sculptra did not help your facial lipoatrophy.  Sculptra is a stimulating agent that normally requires several treatment sessions to gradually produce volume replacement.  Depending on the severity of the facial lipoatrophy, it is possible that 6 or more treatment sessions would be needed.  The amount used with each treatment session would also make a difference in the results.  It is possible that you just were treated completely with Sculptra.  Radiesse is more of an immediate filler, so you would see more improvement right away but the result generally would not last as long as Sculptra.  

Sculptra  produces a subclinical inflammatory response which induces the fibroblasts to produce collagen. Radiesse is a filler and does not require the body to create volume. The two can be used separately or together and in any order. In my experience, Sculptra lasts longer than Radiesse. Find a cosmetic specialist. Ask to see photos.

Both Sculptra  and Radiesse stimulate the production of collagen by your body.  Radiesse is more of a voluminizing agent . It takes time for the collagen effect to work in both cases.