The jury can only find someone guilty of a lesser included offense if that offense is included as part of the jury instruction. Sometimes prosecutors don't want to give the jury that option. Defense attorneys usually try to get a lesser included offense put in to "hedge their bets". If it's not in there, and the jury is of the mindset that they do not want to charge with the main offense, the defendant can walk even if the jury believes he's guilty of something. Likewise a judge at a pretrial hearing can determine that probable cause lacks for the charged offense even if it arguably exists for a lesser offense. If a case gets dismissed by a judge the prosecutor can come back and charge the defendant again without double jeopardy being an issue. But if a jury acquits on 2nd degree murder and manslaughter was not an available option, then double jeopardy bars a subsequent prosecution. (Don't quote me on that, but I'm pretty sure).
This was my understanding of it as well, but when do other possible charges have to be brought forward?