Already joined conference is not accessible smack 4.1.7

Am developing an Android app which makes use of XMPP (Smack 4.1.7) to implement the chat feature. While implementing Multi User Chat am able to send invitation and invite users and start chatting with the group and it works fine until i close the app.

When i relaunch the app and try sending a message to the same group, am getting an error message as,

Conference room does not exist or Only occupants are allowed to send messages to the conference

Am pretty sure am sending to the same JID (groupname@conference.serveraddress).

After i relaunch the app and request for the joined group list it returns null. So am guessing some setting or property should be turned on to store all these details in the server.

Has anyone faced the same issue? Should i add any extra parameter to configure this feature in server / client side?

Thanks in Advance.