There's a bug when adding Repeatable Fields in pages, posts or custom post types. When you click the '+Add' button from any of the above, you get the 'Adding' message but the page just gets stuck there. The new field set does not 'appear' to get added. BUT when you press 'Update' the new set is there.
I know others had a similar problem with Repeatable Field Sets a couple of months ago, caused by the 'TinyMCE Advanced' plugin, but it seems that the updates to the native editor in WP 3.9 are now producing this problem out of the box.
I have tried Chrome, Firefox and IE (all on Win7), with the same problem on all.