Speed storage in VTK files

Dear all,
on the base of the show case example, externalFlowAroundObstacle, I made modifications to use my own STL obstacle. I grown slowly the lattice resolution to get more and more details.
I’m facing a problem which I do not know how to work around: without changing the code itself, with a resolution (Y axis) of 50, 100 everythink work fine, the VTK files are correct and I can visualize the result perfectly with Paraview. But when the resolution is 200, data stored in the VTK files are null or very near 0, but PPM files saved during same run, show correct shape of velocyties.
Does anybody have an idear of what’s happend ?