First: pretty much All Of The Above. You set the situation up that way, and you are learning that that was a bad way to set up the situation.
What you should do now it to learn from that experience; and learn from all of the comments above. [For details of Rule-Of-Three and other stuff, see above.]
Second: [MENTION=177]Umbran[/MENTION] -- I Google'd "Naeser's Law" and came up empty. Does anybody else other than you use that name to refer to [nothing is damnfoolproof]? Could you provide a link where it is called that?
Third: My own original contribution to this discussion:
Bad Carrot: You are using the wrong "carrot" (and you can ignore the stick for the moment) for that particular group of players. They're working toward gaining XP, which comes mainly from "kill-them-and-finish-quests" in the official books; so "kill-them-and-finish-quests"is what the party's PCs mainly do.
So: If you want them to roleplay more, or search more, then explicity offer XP for roleplaying and for searching, and remind them about that offer at the start and and at the end of each session.
Also, explicity "sum up" at the end of each adventure arc, and tell the players the nature, but not the details, of all of the things that they missed. (For a good example, I like Chris Perkins' summation at the end of the Penny Arcade adventure that started in Hammerfast and that led the party to invade the Ambershards -- he told the party just how much they had failed to accomplish, and it was a lot!)
(Of course this is metagamish. It has to be. Don't try to do this during roleplaying.)