After detectives work on a homicide case and cannot solve it, what happens? Do they pretty much forget about it unless a newer case leads back to it or new information just randomly comes to them?

I am mainly referring to the San Francisco police department. One of my closest friends in high school was brutally murdered in 2002. The detective on the case didn't even find me to ask for information until 6 months after her murder!! (I had no info anyway.. otherwise of course I would have sought them out!)

Another friend, was murdered in 2003. These cases are both unsolved. I'm pretty sure the second was because of his involvement with Russian gangs. But there is one page of unsolved homicides on their website and I know 2 of them!

I'm just wondering what the process is actually like. I'm fully aware there is little hope into solving her case, especially over 6 years later. I'd just like to have a different perspective of what goes on behind the scenes-- as opposed to watching detectives on TV solve cases in 45 minutes!