How do I add silence at end of track to allow reverb to continue?

I have a very long reverb added to a track, and I want to have the reverb sustain past the last note, but it cuts off at the end of the clip

...I've tried adding silence to the end of the track, but it doesn't seem to stick...the playhead stops at the end of the clip, and returns to the beginning.

What am I missing? What's the right way to do this?

Thanks.

(latest version of Audition, on a Mac, using iZotope's NeoVerb if that makes a difference)