Using external dsp in mastering

Good afternoon
I am trying to use my x18 dsp in mastering a session.
I have succesfully employed it to process existing tracks, which I then send to the x18 (routed through ardour usb outs), with a stereo XTec eq inserted in 2 channels and then return these to a new track in Ardour, set to record. This works well.
I then tried grouping all the existing tracks to a bus and sending that to the X18 and returning this to the master bus. The music plays and is visible on the master meters, but when I export I get silence.
Am I misunderstanding the routing or mastering mixdown process? Do I need to first record into another new track, or is there another way?
Many thanks

Did you check the "Realtime Export” checkbox when exporting? Like this:

It is important because external DSPs cannot use freewheeling mode, so realtime export is necessary.

Thank you. I tried realtime before, but for some reason it didn’t work.
But trying it again it is working now