Why am I not able to view certain features in the Portfolio Release Planning view even though another user, who has the same permissions in the same project, is able to view the features?
By default a Feature gets created under whatever project you are logged into at the time. So if you created the feature in one project and then moved into another you wouldn't see it.
To find the feature, you can:
1) Use the Search function in the upper right if you know a title keyword.
2) Navigate to the project at the top of your hierarchy, make sure Child Projects is selected at the bottom of the project navigator, and look at the full list of Features on the Portfolio Items screen.
3) Include the "Project" field in your Column list to see where your feature is sitting - it may have been moved.
4) If you still do not see your feature, check to make sure you don't have any filters set.
5) Check the Recycle Bin in the lower right-hand corner to see if it was inadvertently deleted.