DDE connection failing after log off .

Hi,
I am using DDE to access real time data from a control network.The server and client are functioning properly when the connectivity is established after rebooting the the system.
But the client fails to access data as soon as the user logs off and logs in (either as same user name or as some other user).The connectivity is failing at the Point where the API DDEConnect is called as it is returning zero value after log off and log in.
I am using Windows 2000 and MS Excel 2003 as client.Excel is Showing #REF in place of data when logged in afetr logging off.
It seems the conversation value returned by ddeconnect is zero.The callback function DDECallback is non reentrant can this be the cause of problem.

Can anyone please suggest some solution for this problem