Rit Playback

I've cut and pasted the attached last few bars from a quartet score I'm working on. I want the bars in volta 2 to rit and have selected that from the Tempo pallet. I've selected the dotted line and gone to properties but no matter what value I put in the playback tempo variance field, it just slows to a crawling rate which I can't change. I would be most grateful to know what I'm doing wrong. Thanks.