To make wax melts smell stronger, first check that the wax is fresh enough, the warmer is working as intended, and the room suits the setup. Adding more cubes can help in some situations, but it is not the only adjustment worth making.
Wax that has already been warmed through several sessions may have little fragrance left to release. Remove the old portion as directed and try a fresh portion of a scent you know. This is a better comparison than continually topping up an increasingly full dish.
Allow it to warm before judging the result, then leave the room and return. If the scent becomes obvious when you come back, it may already be plenty strong for anyone entering the space.
A small warmer may work well in an office and feel faint in a living room that opens into a kitchen and hallway. Consider the area the fragrance has to fill, not just the room where the warmer sits. Keep the warmer in a suitable open location on a stable surface, following its required clearances.
Notice whether a strong draft carries the scent away from your seating area. You can evaluate another safe location without sealing up the room or putting the warmer beside flammable materials.
If you prefer a bold fragrance, read descriptions for intensity as well as notes. Your own experience with similar scents is useful, too. Increase the wax portion only within the warmer's capacity, and never substitute a higher-wattage bulb or another heat source to force a stronger result.