Dear Diana,

I would say that Mustafa is probably correct, at least there is something wrong with the headers of your files.

If the file extension is .nii, make sure it's in the correct NIFTI version (not ANALYZE saved as .nii file, this is different). To make sure they are generated correctly, you can convert from your DICOM using SPM dicom to nifti integrated converter, or also MRConvert works nice for me.

Hope this helps,
Best regards,
Stephen Larroque