Chat Messages dropped during conversation


I have set up 2 openfire servers. 1 server is in the Head office and the other is in a Branch Office. The 2 servers are connected through a VPN tunnel so there are not communicating over the open internet. We are using Spark as our chat clients.

When 2 people are chatting, 1 in the head office and the other in the Branch office. Sometimes some messages are dropped and dont appear on the other side.

So what could be the Problem?

I have all the openfire server setting to default, because when i first installed both openfire servers, they just worked easily so i didnt make any changes.

So are there any settings or other optimizations i have to make?