asp.net 4.0 - When I create a custom control that inherits from HtmlContainerControl, I can't get it to show up in the VS Toolbox

I've tried two methods to include my custom control in the toolbox.

First, I tried to drag the DLL onto my toolbox, but only some of my controls appeared.

Again, when I did it the "choose items" way, only some of my controls appeared.

Any of my custom controls that inherit from Web.UI.HtmlControls.HtmlContainerControl don't show up.

However, if I change those controls' classes to inherit from Web.UI.Panel instead, they do show up.

What am I missing?

Thanks