How do you know they're not still fighting? Are you monitoring them constantly? If you've got 4 girls chewed up and one that you know to be dominant spot free, I'd say it's pretty likely she's still picking on them.
What kind of filter are you using? If you don't have it prefiltered the intank tube could be causing issues. They could also be shredding it on things.. have you done the nylons test?
Have you tested your water or ammonia nitrites and nitrates? What is the size of the tank and how often/how much do you do a water change?
I know sometimes put sororities in smaller tanks, but at least 20 gallons with 6+ girls seems to work out best. Bettas need to be added all at the same time, and potentially cupped, held and repeat.
Have you seen the sticky that dramaqueen wrote? http://www.bettafish.com/showthread.php?t=29402