Wavelab with Avid HDX

I use Wavelab 12.0.51 with Avid HDX hardware and Avid Converters. The main output works, but problems begin with the Plugin “External Insert”… I can see a signal on the plug in´s output, but nothing comes in at the return of the plug in. With HDN it works.
It was the same with Wavelab 12.40. Any idea?