Sounds for me like you haven't yet killed all of the target pirates.
Mostly such a target has jumped out system while you have done some dogfights with some other pirates.
I you have done several bounties like me, you will see sometimes a ship heading straight to the jumppoint while other ships start fighting you. Mostly this single ship is your real target while the other ships are only some kind of protection escort for this ship. If you are not able to destroy this ship before it jumps out system, you will have to jump in this system after destroying the protecting ships.
For example if your target is in Regallis at the jumppoint to Freya and you start fighting the pirates, one enemy ship might jump to Freya. If this was your real target you will have to jump to Freya too to shot it down.
A short HowTo:
Check you Nav computer if the target is in system. If not, jump to one of the other mentioned systems and check Nav computer again.
If target is in sytem fly to the Navpoint of the target.
If the navpoint is an jumppoint, start the fight and look if one ship is trying to jump out system. After killing all enemies check your Nav computer if the mission is complete. If not, jump to the system where one ship might have jumped to. Check this system for the target and shot it down. Check computer again, mission should be complete. Otherwise you have not yet found the real target in the system. Try next navpoint until you found your target.