Hi James,
Are the invitation emails nested programs (email program type) within the tradeshow program (nested email program is available for the event program type) or sent via a batch smart campaign in the tradeshow program? If the former, the entry type would be "Email Program" and If the latter, it would be "Smart Campaign with Email", so check that the type associated with the missing invitation is included in your calendar filter. Also if it is a nested email program and are filtering by program > channel, be sure to include the nested program's channel, which could be something like "Email Blast".
Totally different thought, but is there a "X more..." note in the square for date where the missing invitation should be? If you click that it should show the full list of calendar items on that day on the right hand side of the screen.
The custom entry type tags are used to categorize non-Marketo marketing efforts that you would manually add to the calendar.
Hope this helps!
Beth
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