Hello Rashmi,
You have mentioned that the issue does not occur for all the users.
So please follow the below steps:
1. Check if the webservice Id of the users for which the catalog is working and those for whom the catalog is not working are the same.(Yes. this check is the most basic check).
1.1 Uncheck the "Use HTTP GET" check box and re-login to your SRM Portal or NWBC after closing all sessions
2. If the webservice Ids are the same, before launching the catalog,
2.1 Press F12,in case of Internet Explorer, go to Network tab and press F5 or click on green PLAY button(applicable only for version 10 or above). Else use Fiddler or HTTPWATCH for network traces.
For Chrome, just F12 will do
3. Launch the catalog and check the traces for the parameters passed to external catalog.
Revert in case the issue persists.