When you say that you must still hold your stomach in to gain the desired look suggests there is something wrong with the muscle tightening component of your tummy tuck. It is possible that not enough tightening was initially achieved or that the sutures responsible for the tightening gave way. Repeating the muscle suturing is safe and may be the answer.
However, if you are harboring a significant quantity of intrabdominal fat then no amount of muscle tightening will help. Intrabdomiinal fat is stored around the stomach and intestines and often times goes unrecognized. The bulk this fat creates prohibits the surgeon from maximally tightening the abdominal muscles during tummy tuck.
The good news here is that if this is the problem you don't need nor will be helped by additional surgery. The bad news is that you are going to have to diet to lose this intrabdominal fat to get the desired look. Your surgeon should be able to determine the correct course of action.