Sometimes you're just going to have a wimpy workout. Either you're exhausted from a long day, week or month, you're out of shape because you're just getting started, or you have been down sick for awhile. Whatever the reason, there are times when your workout will feel less than stellar.
During those tough workouts, just do it! Remember that in order to get a good workout, all you have to do is push your limits. If those limits are a little more "limited" today, you will still get a good workout at that lower level. Just do your best and you'll soon be back up to your old level, and in the meantime you'll burn a lot of calories getting there!