Yes please check on what boot mode you are on,
For simplicity, could you please first check if tftp works or not ?
On target:
#tftp 0x800000000 uImage ?
If this works then you need to modify the bootargs and bootcmd.
You can refer to the followings:
http://processors.wiki.ti.com/index.php/AM335x_U-Boot_User%27s_Guide#Environment_Settings_for_NFS_Filesystem