The first thing you'll want to check is to make sure that the cards on the board you connected via Cronofy have due dates assigned to them. Dates added using the Custom Fields Power-Up won't display on an external calendar.
Also, to clarify, Cronofy will allow you to view due dates from multiple Trello boards in a single external calendar. It won't combine due dates from multiple sources in the Calendar within Trello. For that, we'd suggest the Planyway Power-Up.
If the cards have due dates but aren't showing up on your external calendar, I'd suggest either contacting Cronofy's support (via the chat icon in the bottom right corner of http://help.cronofy.com/), or sending us an email at https://trello.com/contact, so that we can take a closer look.
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