If the size recommender is completely absent from your product page, it is being hidden rather than broken. Work through the four causes below in order — each one is quick to rule out.
This article is for when the recommender does not appear at all. If it is showing but giving a wrong answer, that is a different issue.
1. Your plan
Recommenders are available on Plus and Ultimate plans only. On any other plan, nothing renders and no message is shown to the shopper. Check your current plan and upgrade if needed before investigating further.
2. A setup warning
If the recommender setup has any warning against it, the recommender is withheld from your storefront completely. To check:
Open the size chart.
Go to the Recommender tab.
Clear anything flagged there.
Even a single unresolved warning is enough to hide the recommender entirely.
3. Where it is being placed
If the placement selector does not match your theme, the recommender falls back to appearing just after the size chart. If the size chart itself is not appearing either, fix that first before returning to the recommender placement.
4. Chart matching
Confirm the chart is actually assigned to this product. The quickest check: verify that the size chart itself appears on the product page. If it does not, the chart is not matched to the product and the recommender will not appear either.