Sometimes people ask me, why do I see those lines. The answer to that question is, you left the wrinkle way too long before any repair. What happens to the carpet is, the wrinkle not only creates a permanent damage to the underside of the carpet, but also dirt and grime gets collected in the area. The wrinkle also prevents the vacuum to do its job. Plus everytime someone steps on it, it gets damaged.
After I am done with the stretching, the carpet will be flat, but you might see the line.
If you look at the examples below, 1st picture is before, and the other two are after the stretch is done. Carpet is flat at the end, but you will see the line.