Rashi: They stray on all the mountains. I.e. they became Hefker; everyone does like he wants, and you did not rebuke them nor judge between people.

Malbim: There was still a possibility that they will return by themselves to their place and gather. However, via being scattered they erred to ascend mountains, and from there to the hill ? "Tzon Ovedos Hayu Ami Ro'eihem His'um Harim Shovevum me'Har El Giv'ah Halachu Shachechu Rivtzam" (Yirmeyah 50:6). Via this, they forgot their place ? "Al Pnei Kol ha'Aretz Nafotzu Tzoni", for they forgot where they lie.

Malbim: When the flock is lost, the shepherd goes to seek them. When he does not find them, he investigates and asks if people found them; if so, he can give a Siman via which they will return them to him. Not only does no one investigate ? no one even seeks them.