UltraStudio Express with FCPX

Hoping someone here has figured this out...

I use a MacBook Pro and an UltraStudio Express as a portable editing solution. All works well, except... I can't always see FCP's output from the BM box. FCP recognizes the box, and "AV Output" checks but sometimes there just isn't a picture.

Sometimes a combination of unchecking "AV Output" and rechecking it works. Sometimes unplugging the Thunderbolt cable and re-plugging it works. But I haven't found any guaranteed method.

The output always works with Media Express, but I'm often working on a very quick turnaround and the extra step of rendering a master file and opening Media Express is inconvenient. I should be able to just play it from my timeline.

Be sure yo have the latest updated driver from BMD.
What versions of FCPX/OS X?

I was on Desktop Video 10.4.0, upgraded to the latest and still no joy. Really bumming me out.

FCP 10.2.1
OS 10.10.3
Desktop Video 10.4.1

I've used BMD and AJA products for many, many years. They are both notorious for releasing mega-buggy drivers. I'd contact BMD tech support and see what they have to say.