You are only offered chat once per browser session. If you decline the first offer, it will not be presented again.
Although we are working towards chat-on-demand, the functionality is not available.
There are variables that change, but essentially, looking at Knowledge articles, and just time on the site should offer the chat dialog after you start a new browser session (i.e. close all your browsers and log back in to
https://support.avaya.com)