Most, if not all, regions in Neverworld Grid run on newer versions of OpenSim utilizing UbODE physics engine. Newer versions of OpenSim and especially in combination with UbODE require all mesh to obey the collada standard commonly used for mesh in OpenSim.
Mesh uploaded on previous versions of OpenSim, which allowed for improper collada - not to standard, often exhibits strange physics behavior, because part of the required data in the mesh is missing. Unfortunately the only option to resolve this is to upload a proper copy of the mesh.
If you are having issues with something you brought from other grids or bought somewhere it is best to contact the original author for a fixed version. Alternatively meshes can be downloaded, if permission allow, and fixed using a 3D modelling program such as blender.