RHEL 5.6 automount Solaris NIS maps didn't work as expected

NIS-Master: SUN Solaris
NIS-Slave: RHEL 5.6

NIS-Client ( bound on NIS-Slave !) ; RHEL 5.6

automount is only working partially.

It doesn't mount auto.master entries at all - which are NIS maps too ?!
Any ideas, comments, hints tips ?!

Regards