Geometry nodes - issues with collections


I have a geometry node set up on a plane, intended to create subsurface scattering of rocks.

When I point instance to an object, no issues. I duplicated the object originally instanced (did not move it or anything), and created a collection, now with two rocks. When I point instance the collection, the rocks do not stick to the plane, they are scattered crazily along the z axis.

Thoughts on what i’m doing wrong?

Many thanks.