Workling a muscle too little?

Is it ok to work the same muscles again the next day if i'm not sore? I have no idea why i'm not sore for ex. Curls(machine) 3sets of 8 with 25 Lbs. i can barely lift the last 4 or 5 should i be adding more weights or less weights so i can do the last 4 or so?
 
ohhhhh, ok
well then... is it ok to do full body 3x a week? lol, i like going to the gym, even if its just to run. and... what are good abdominal excercises? cuz i just do crunches when i get there 3x a week
 
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