After a year and a half post op, muscle repair failure. What can do I about this?

The best and first test to assess your bulging abdomen is to get on the scale. if you don’t remember your exact weight at the time of surgery then get copies of your medical records which will include your weight since that’s part of information gathered for surgical procedures.
If you’ve gained weight then you need to lose the weight and see if that has an impact.
Delayed muscle repair failure is virtually unheard of.
Midline abdominal incisions can develop ventural hernias but that is quite different than having had muscle tightening after a tummy tuck. at 18 months scar tissue is holding tissues together more than sutures are. surgeons do not rely on sutures to maintain strength for a lifetime when doing it surgery but instead rely on the bodies scar tissue to hold things together. Even the permanent sutures will eventually lose their strength.
Including a picture would also help in making some kind of an assessment.
Excess visceral fat due to weight gain is the most common reason for a change with increased abdominal bulging after a tummy tuck.
It would be especially helpful to see before and after pictures to see if there was presence of excess visceral fat on your before pictures.
If there was then that is most likely the explanation.
You can also follow up with your provider who can examine you and give you an assessment. Likewise you can also have a second opinion consultations with other local board-certified plastic surgeons. Without pictures we really can’t give you any quality assessment answer at this point.
Best,
Mats Hagstrom MD