My guess is luff entry angle and luff shape. Your shape might be to flat in the front, so you have a very narrow 'groove' where the spi flies as you want it.

It's really hard to tell from the pictures. It would be easier if you did some photos from below it and straight upwards. Even better if you had some draft strips on it (I know, not good looking on a spi). It's important that you have it sheeted as you would when sailing when you do this.
If you hoist your old spi and take some pictures from the same angle with the same settings for sheets etc, it should be easy to compare luff shapes. Accumeasure from UK Sailmakers (downloadable from their homepage) is a nice tool for comparing draft and shape trough pictures.