There are certain situations in which a skill is not available (against a particular enemy on a particular tile) until the height level is adjusted. In the example in the attached screenshots, the tile of the desired target is visible before and after adjusting the height level, but the tile is only marked in red as a potential target (after selecting the skill) once the height level has been lowered. This gives the impression that the desired target is not actually a valid target, when in fact it is.