You found the location of the monster that you wanted and ran there ready to pry an item of legendary prowess from its cold, dead paws. Hmm, it's not here. There is some other scrawny monster here. What's going on?
Monsters that have a fixed location may have placeholders: monsters that 'cover' for them when they are away. To get to the monster you want, you will have to kill its placeholders first. Maybe once, many several times. The spawn timer determines how long you have to wait before another placeholder or the monster you want appears.
Some roaming monsters share a spawn table with other monsters. To get the ones you want to hunt, you may have to kill other monsters in the area. Taken to the extreme, this is the Scorched Earth method of getting rid of placeholders to get the monster you want.