Valerie Carter was a backup singer for James Taylor during the 1970s and 1980s. She was known for her powerful and distinctive voice and contributed significantly to the sound of many of his albums.
There is no public information available that suggests they stopped working together for any specific reason. It is likely that their collaboration naturally evolved over time, with Valerie Carter pursuing her own successful solo career while still occasionally appearing as a guest vocalist on James Taylor's recordings.
It's important to remember that the world of music is dynamic, and collaborations often shift as artists explore new paths and pursue their individual creative endeavors.