I am using InDesign CC (13.0) on Mac OS Sierra 10.12.6 using a MacBook Pro13 in, mid-2012.

When The Buttons and Forms panel is docked with the other panels, it will usually refuse to expand. There does not seem to be a set pattern to it, but it seems more likely to expand correctly when working with TextFields than when working with Radio Buttons, Check Boxes, or Combo Boxes. I have seen it work correctly when working with a Text Field, then when I select a Radio Button, the Buttons and Forms panel will momentarily populate with the expected content before blanking out. But usually, it manifests by simply remaining collapsed, as though there is no content to show in that panel. When this happens, the panel menu options are also inactive.

I eventually found that if I un-dock the panel, it does show the expected panel content.