If a product you expect to see isn't appearing in the comparison picker, check the following.
If the Scope setting in Settings → Compare Visibility is set to Same Type, only products of the same type as the current product will appear in the picker.
Verify the missing product's Product Type in Shopify matches the current product's type exactly.
Check whether the missing product has an Exclude tag applied, or is missing a required Include tag.
The missing product's type must be listed in the Allowed Product Types field under Settings → Compare Visibility.