The skill landing and the item being awarded are separate questions.
An item's spoil acquisition table identifies recorded monsters, quantities and published x1 item chances. It does not measure the initial chance for the Spoil skill to mark the target. A successful mark therefore must not be interpreted as a guarantee of every item listed for that monster.
Use Spoil to place a living monster into the state that allows extra collection. After it dies, use Sweeper on the spoiled corpse. These are separate actions with separate purposes: Spoil prepares the target; Sweeper collects from the qualifying corpse. Simply killing a monster listed in an item’s spoil table is not the same as completing that sequence.
Start from the item’s spoil acquisition tab and compare the listed monster, level, locations, quantity range and x1 item probability. A rarer result with a larger stack can still be less convenient if the target is slow to kill or difficult to reach. Check the effective server-rate display separately from the base record. Neither column establishes your chance to land the initial Spoil skill.
Check the entire sequence before changing hunting areas: whether the target was successfully spoiled, whether the corpse was swept, and whether the desired item is actually a spoil result for that monster. A normal drop row is a different acquisition method. Even a valid spoil entry describes a possible result rather than promising that item after every kill.