You prob. may very well be the fact that you are not doing enough cardio. You need to burn off all the fat around the midsection before your abs will show thru. Be careful doing the weighted situps because the added weight will build bigger muscles not necessarily more defined muscles. If you do too much weight you will create a bulk type midsection. YOu don't want large ab muscles you want them lean. I would suggest losing the weight and doing MORE reps and MORE sets and working a lot more cardio. Also, keep track of your diet. If you eat a lot of sugar that will have an effect as well.