issues with Fsl fast segmentation

Hello experts,

I am trying to segment WM, GM and CSF in neonates lamb brains using Fast. However, it fails to segment the tissues correctly.
I used different options, reduced -R, with no success. attached are the segmentations I get using the below command:
fast -S 2 -o t1tot2 t1tot2.nii.gz T2-56NL-masked-stride.nii.gz

Could you please let me know what do I need to change to get the correct result?