The snag with the roll-your-own roundabout is that Waze will likely not direct you to 'enter roundabout' or 'exit at ..' or proceed straight through and will likely get confused with the routing as well.
The easier option if it keeps flaking out on you is likely to kill the existing roundabout and simply redo it.
Delete all ROUNDABOUT segments only, save and check for errors, and then drag and align the road segments you want to connect as close as possible to the center of the roundabout and create a new one from the menu, or press 'O'.
As long as the connecting road segments are not deleted, Waze should retain their traffic info.
When a roundabout is created, WME 'fixes' the default routing based on the connected segments, so if all directions are indicated correctly, it should be created fine.
You can also verify this by clicking each connecting segment of road and each segment of the roundabout.
Full instructions at http://www.waze.com/wiki/index.php/Map_Editing_Quick-start_Guide#Create_a_roundabout
[Edit] Oops, forgot to mention.. Each segment of the roundabout should be named at least with Country, City and likely State in your case. The roundabout 'Street Name' should not be filled in.
If It's not named, it will leave another horrible red 'Blot On the Landscape'.