For future reference, when bounty hunting, it's generally a good idea to, once you're finished, head back to a station within that same system and cash in all the bounties you have. Basically, anyone you scan with the basic scanner that you get when simply targeting someone, who shows up as "wanted", is wanted by a faction that operates within that system. So if you leave a system with un-cashed bounties, and try to cash them in elsewhere, the faction that put up that bounty is likely not present within that new system, so you can't cash it in.
A Kill Warrant scanner lets you scan for additional bounties that are placed upon a ship that has been issued by factions outside of that system, which is how you can end up with a bunch of extra bounties that you can't cash in within whatever system you're in when you find them. For these extra bounties, or any bounties you collected with the basic scan but then didn't cash in, you basically have to leave the system and either track down the individual factions that issued each bounty (a massive waste of time, IMO), or go to an Interstellar Factor.
Interstellar Factors are places where you can cash in any bounties, regardless of where you found them and who issued them. They basically act as middle men, taking a 25% cut of the profits to facilitate the transaction. They're usually found in low security systems. You can also filter for them on the galaxy map, though I can't remember if these guys show up properly or you have to visit them first to make them appear. Either way once you find them they're very useful. Don't bother trying to track down every individual faction if you have a few outstanding bounties, make life easier for yourself and just find an IF.
EDIT:
Interstellar Factors. They also let you pay off fines and bounties on yourself, so I'd suggest finding a couple and keeping them bookmarked for the future. They're pretty handy.
