Zero values across the domain

Hi,
I created a file from my FreeFEM code by iterating to produce files such as “C_500.vtu”.
However when opening the file up on Paraview, it shows zero across the entire mesh. I know that the variable I am plotting is non-zero from plotting using FreeFEM.

Does anyone know why this may be happening and how to fix it?

Thanks,
Alistair

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