Why isn't the iOS Photo Mass Delete Techinque working on older photo events/albums?

I

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iOS Photo Mass Delete Techinque not working on older photo events/albums?

When using the steps described here: http://www.imore.com/how-quickly-delete-photos-and-videos-iphone-or-ipad some of the albums/events won't delete. When the albums/events are selected in the "Moments" view the trash can doe not turn selectable?! Seem to happen with older albums vs. ones I just saved recently. What gives?