Circumstances can make Morale checks harder for a unit to pass. For example, a unit that has already suffered heavy casualties will be more inclined to give way than a unit that is untouched. This is represented by Leadership modifiers for Morale checks and these reduce the unit's Leadership value by -1, -2 or sometimes even more.
Leadership Modifiers
Source: Core Rulebook 3E, p. 70
Circumstances such as heavy casualties can reduce a unit's Leadership value by -1, -2 or more for the purposes of a Morale check.