Why don't the "Prices drop" or "On sale" filters show any results?

The On sale and Prices drop filters check different information.

On sale: the product flag

This filter uses the product option that requests the “On sale!” flag. In the back-office product form, open Pricing, enable the option for displaying the “On sale!” flag and save. Its wording can vary with your PrestaShop version. This flag can be enabled even without a price reduction; how the badge appears depends on the theme.

Prices drop: a specific-price reduction

This filter selects products with a specific price whose reduction is greater than zero, whose validity period is current at indexing time, for the engine’s shop. The specific price must apply to the product without being restricted to one combination: choose all combinations if the product has combinations.

A fixed price without a reduction, a future or expired period, or a reduction restricted to one combination is therefore not enough for this filter. This filter does not simulate every condition of a shopping cart.

Also check that the product is active and included in the catalogue indexed by the relevant engine.

After a change

Re-index the relevant engine. For scheduled discounts, also arrange a re-index after the start or end of their validity period. See the re-indexing methods. If needed, then use Maintenance > Clear cache in the module.