Here's why:
Option A - in case you're using the custom stock or default stock options
In this case, your app is not setup properly.
Follow the placement guide to add it to your store.
Option B - in case you're using the real stock option
In this case, it's a result of your total setting.
Follow these steps:
(1) Add the total to be the current stock level + 1 of your most in stock product
(2) Mark "show per variant"
(3) If you used the "hide on large stock" option - make sure you're not trying to test a product with more stock than your setting.
Have you verified (1)+(2)+(3) and saved your changes?
In this case, your app is not setup properly.
Follow the placement guide to add it to your store.