Collider behaviour

As I was building a simple scene, I came across a strange problem.
I’m dropping things into a playing area from a small height and have all the objects using gravity. None of them are trigger colliders but the objects using a mesh collider were dropping through the floor where the box colliders were landing and staying where they should on the ground. I changed the mesh to a small box collider and they fell through again.

Eventually I fixed it by changing the objects to spheres, as the sphere collider also stayed on the playing surface instead of falling through.
Anyone know why this was happening?

Were the mesh colliders set as convex?

Hmmm… I don’t think so.
Would that have caused something like this to happen?

I think it could have caused this if they weren’t. Moving triggers should have a rigid body attached and if it’s a mesh collider, it could make things easier for it if they are marked as convex.