Hi,
Sometimes, when the characteristics data is inconsistent in the database that can happen.
In that case,you need to :
1. Delete all the variants of the product on the listing of variants that you can access via the button "manage variants" on the product edition screen.
2. Delete the characteristics of the product.
3. Save the product.
4. Add the characteristics again.
5. The system will regenerate all the variants automatically and you should be able to see the dropdowns properly on the front end.
If that's not the case, please provides links to a product page where you have the problem.