NPC movement problem on slopes

I have some NPCs using Unity 5's AIThirdPersonController that comes in the standard assets. For some reason, they can't climb even moderate hills (using the built-in terrain) even with the navmesh allowing the max slope of 60 degrees, and despite the fact that the player character can climb hills just fine. When the NPCs attempt it, they slide around as if they're on ice, often sliding all the way down and across level ground; and they also sink into the terrain although they usually remain on top of it in other circumstances. Any idea what causes this?