cmd.exe at \computername to run on remote machine

My original problem was the NirCmd remote \PC_D ... (http://www.nirsoft.net/utils/nircmd.html) was failing. I tracked that down to it using at \computeame command which is not working on all machines. The local use of at time ... works fine on all machines. I have three PCs (let's call them PC_L,PC_D,PC_H) and two of them are getting this error from testing in cmd.exe:

> at \computeame
> Access is denied

That shows \PC_L is setup correctly but the other two are lacking something. Something with \PC_H and \PC_D is not setup properly (ruing XP SP3 home and XP PRO edition respectively). Testing shutdown in cmd.exe:

> shutdown -s -m \pc_h -t 9999
> The network path was not found.

This error seems puzzling. So to summarize:

FROM    FROM  TO    AT      REMOTE SHUT
TYPE    HOST  HOST  CMD     CMD
======  ====  ====  ======= ===========
XPPRO   PC_L  PC_D  FAILS   OK  
XPPRO   PC_L  PC_H  FAILS   FAILS
XPPRO   PC_D  PC_L  OK      OK
XPPRO   PC_D  PC_H  FAILS   FAILS
XPHOME  PC_H  PC_L  OK      OK   
XPHOME  PC_H  PC_D  FAILS   OK  

I have checked the following:

• All machines are logged in with a the same user name with same password, and members of the Adminstrators group.
• Access file shares works fine.
• net view \computeame works.
• All machines are in same Workgroup name.

What other settings should I check?