Cross platform chatting has some issues

Hello Team,
Thanks for providing a platform where I can develop chat application easily.

I have created a chat app which is working fine when I am chatting with similar platforms like android to android, iOS to iOS but in cross platform it is not working. No message is sent when I try to chat with android to iOS.

What can be the issue??Does it has anything do with any configuration file or server specific settings???

Please suggest.I dont have in depth knowledge about it.
Thanks in advance.