There are many factors that could cause a failed treatment of this type. The exact shrinkage that takes place depends on how much heat is applied and for how long. Not enough heat may not shrink the collagen fibers. Too much heat can actually kill the tissue, a condition called heat necrosis.
The extent of the damage before treatment is important, too. For example larger tears are less likely to respond to this treatment and more likely to tear again. Smaller tears may respond better but studies show long-term results (five years later) aren't successful. The collagen shrinkage doesn't hold, and the ligament becomes lax again.
You may be better off having a surgical repair. There's less chance for reinjury and degenerative changes in the joint. Talk to your surgeon about treatment options at this point in your recovery.