Generate mesh of same model. Success in "coarse", but failed in "medium"

Dear All,

Is there any limitation of nodes? or any other things go wrong?

Coarse:

"_mesh 0.075 256 False
Generated 4505 elements, 7058 nodes"

Medium:
"_mesh 0.05 256 False
Meshing failed with the following error:
Fatal: Could not generate mesh."

Thank you.

Regards,

William