Why would a completion pop-up appear disabled?

I'm trying various things with the C# completion provider and there are times where I get it to pop-up with a gray background (as opposed to white), and clicking on items in the pop-up doesn't select/insert them. Based on the visual and its behavior, I'm assuming the completion provider is in some sort of disabled state. Can you explain the possible reasons the completion pop-up would appear but not be enabled?

Thanks,

-Craig

[Modified at 08/30/2011 02:59 PM]