Igu
This shouldn't be the case, are you sure you unlocked the chapters with spirit-stones?
Sometimes when readers check back into previous chapters, they will realize that it's locked, this is because translators/webnovel extends the paywall into the older chapters, and people generally forget that they read it when the chapters were NOT locked, meaning they didn't actually purchase the chapters.
Are you also sure that the novel wasn't on limited free reading time?
Lastly, can you give me the name of the Novel?