When I switched from registax 5 to 6 - one simple thing that caught me out is you have to click the 'set alignment points' tab at the top left after loading the avi. Then you slide the slider across until you've got somewhere from 60 to 100 and let it go. I've found for planets the defaults are pretty good. Of course, if your data manifests excessive awesomeness, you may need to apply an appropriate pre-filter
cheers,
Andrew