Do they have a context assigned? Also, if assigned to a context or project that is on hold or dropped. Another possibility (if not in this particular case) is that the start date is still in the future.
The only projects on hold are the few that I use as templates or the many ideas that I keep in a Someday folder. The template projects include start and end dates, too. Could these items be the ones that are indicated by not appearing? It doesn't seem likely because together there are dozens of items in these projects on hold and I only notice this discrepancy with a few items.