When testing a manual move of a SharePoint 2007 Enterprise calendar to SharePoint Foundation 2010, we noticed that several items were missing from the calendar view in 2010, but present when viewed as a list (the "all items" view). I extracted the manifest.xml file as described in this most helpful blog post and compared two records: one that displayed on the calendar view, and one that only displayed on the all items list view.
The discrepancy was immediately apparent:
The imported items that show on the calendar have a defined <end> property (an end date), but those that only show on the list did not.
Spot-checking the data showed these as items that had no duration, but were points-in-time items.
I applied this simple solution:
1. Open the calendar in the datasheet view.
2. Filter the list on the End Date field, showing only blank records.
3. Select the entire set of Begin Dates and copy.
4. Select the entire set of Blank End Dates and paste.
And thus all of these items now display on the calendar view.