In a recent 6.5 build I ran into the problem where the add component button in the sitecore page editor wasnt offering any possible placeholders in the ui. I’d setup placeholder settings so that the item names tied up to the placeholder keys in my layout.
The expected behaviour would the ‘Add to here’ flyout is available as shown below:
The difference in the newer versions of sitecore is that Placeholder settings template have a new field ‘Placeholder key’:
If this field is blank, the page editor doesnt know how to pair up the placeholder keys and settings. Note, historically this was done via the items name.