Collision failing out of the blue

Hello there,
i have a Problem, and already searched through most of the Forums to get an Answer. But until now, nothing worked.
The Problem is:
I Have a bigger 3D Text (2 Parts), which is falling onto a simple Plane and at the Point where it is touching the Plane there is an Emitter (Half a Circle extruded), which is throwing out little rocks as if they get loose from the text. The Plane has Collision on, BUT the Rocks, which are shooting out and falling correctly, land Beneath the PLane, where they lie as if the Plane would be 1-2cm lower. (So they are behaving “normal”, but as if the Collision of the Plane was a bit under the Plane) And Therefore, you cant see them once they have fallen through it…

Anybody an Idea how to fix this?