Lack of soreness the next day doesn't mean that you haven't worked the muscle....
Personally,i find crunches on a swissball don't stimulate my abs at all....I feel it more in my lower back.And whilst planks will build core strength,i have never found them to change the appearance of my obliques as they're an isometric exercise(depending on how they're performed).
I work abs twice per week
1)-Hanging leg raises supersetted with incline Russian twist.
2)-Crunches supersetted with cable side bends.