Dont have much experience or time in but it sounds like maybe it could be poor manufac construction. I would have a certified carpet inspector look at it. or maybe call the manufac about the issue and they may have an inspector come out. After all most underlayment issues are due to manufac construction.
Charles was it powerstretch to begin with . If it was possibly they did not stretch i good enough. Another can be 1 of 2 things to little or to much latex.
As Harry stated above.. just because someone came out to restretch it does not mean it was done right. Knee kicking it to begin with and the knee kicking again does not solve the issue. If you were able to take as much as you did out and the carpet is not that old, then it was not powerstretched to begin with.