What is the problem ? You are probably spoiling mobs with low chance for items but it doesn't mean they are not spoiled.
with spoil you have two different things: spoil skill landing and spoil chances for items.
For spoil skill landing, it depends on your lvl and mobs lvl. As they said before, make sure you get the message "spoil condition has been activated." This should happen almost all the time with spoiler 68 and mob 70.
When the mob had the spoil condition activated and it dies, then you start considering chances for items (you level doesn't matter for this part, you can sweep lvl 85 mobs with lvl 15 spoiler). Since most spoils are not 100% some mobs might not turn blue. You can sweep them but no item will come.
For the lvl u say and the item I think you are spoiling bone slayers (or more mobs) in fod. That mob for example, its best rate (in a 3x server) is 1 in 5 for dc helm part. This means 4 times out of 5 it will not even go blue containing this item (the other have lower rates, so just consider this one as example).
If you are spoiling mobs with low rate for spoil (you can check this on the mob's hp window here), most times it will not go blue. Remember it's 3x only.