Why is "My Sweet Audrina" a stand-alone novel?

V. C. Andrews wrote many novels and inspired many more after her passing. Most of her novels followed a specific five-book pattern where the reader would learn of the main character%26#039;s identity, then the offspring, and finally a prequel of the character%26#039;s parentage. One of my favorite novels that she wrote was %26quot;My Sweet Audrina.%26quot; Why were there no sequels or prequels to this series? I%26#039;m looking for factual information that could be backed up with references. Thank you.