It's not clear if you're referring to the macOS or iOS version of StitchBuddy, so I'll include instructions to test / restore In-App purchases for both:
Complications might be caused by a different AppleID - you need to be logged into the App Store with the ID which has purchased the product. And purchase restoration doesn't work for customers who have purchased a license from me (before it was offered through the store) - just contact me directly.
Please send me a mail if the mentioned instructions don't help: I can guide you to rather more information about the purchase checks (at least on macOS)…