Not all native tribes were skilled hunter gathers. The Corps of Discovery encountered natives who were starving. Some tribes lived off a single main food source such as salmon. If that food source dried up they didn't have the skills take other game because they never had to before.

The smartest tribes would move from one food source to another but conflicts with other, possibly stronger, tribes could keep some tribes from moving around too much. There's also the simple fact that every time you move you're taking a risk.

This is what I know off the top of my head. I hope it helped.