There are a number of academic papers on research into why LS powder is degraded during the process, and why you need to blend used with new to re-use in the sintering process.
In it's simplest form, the material is held at too close to it's melt temperature for too long to avoid degradation. The changes in the materials melt viscosity make it unsuitable for the sintering process again.
I have used old LS nylon powder for low quality injection moulding, but the fine powder form makes it a challenge in standard moulding machines. Any cost to change that form [extrusion-pelletising] make it uneconomic to recycle.

Hope that helps,


Rich