Please note that the chat log is set to show only chats from today by default, and the chat log shows only chats from when they were initiated the first time, which may have been a while ago; so here, change the filter to search chats for say 'This year' and choose channel 'Internal' and press 'Search,' then you will see chats that where initiated between internal extension in the last year, from there find the chat log between the extensions you would like to see the conversation for and press the message icon on the right of the log to reveal the enter conversation between the extensions.