Marcus21 you could see your Spirit Stone history if you tap/click on your SS balance in your profile.
You'll see deposit and consume history, tap on Consume and you'll see if the SS were spent via unlocking chapters, or if they just plain expired.
As Pizz mentioned, free SS expires after a limited time, even if you don't use it.
IIRC, only takes 30 days for earned SS expire, day-by-day.
Different rates apply for SS gained from purchase.