You could be using the wrong nsswitch template for /etc/nsswitch.conf. During setup, you should be using /etc/nsswitch.files as the name services switch template. After setup is complete, you would then want to use /etc/nsswitch.nis.
Do the following to verify that you are using nsswitch.files:
# head /etc/nsswitch.conf
If you are not using the nsswitch.files
, copy it over as shown below:
# cp /etc/nsswitch.files /etc/nsswitch.conf
Run the ypinit -m