Hi Melanie,
Collisions will be evaluated a certain amount of times per frame. If the objects are fast or get pushed toward the floor, the speed can be higher than the “samples”, and then they seem to fall through or get stuck.
In reality, we have a nearly infinite number of tests when a stone falls to the ground. This can’t be done with standard computers in a reasonable time. So things will be evaluated just a few times.
You can change that: Attribute Manager> Mode> Project> Dynamics> Expert
https://help.maxon.net/c4d/r25/en-us/Default.htm#html/DYNWORLDOBJECT-WORLD_GROUP_EXPERT.html#WORLD_SUBSTEPS
There are other parameters, like the correct scale and collision margins. All of that can’t be analyzed from an image that would require exploring your setup. This would require a scene file. If it starts, for example, in a way that it can be solved at all.
All the best