A loose wall to wall carpet will not last very long because the primary and secondary backing will de-laminate and the carpet will lose its stiffness or tension. This is due to poor installation more often than not.
Synthetic carpets don’t shrink, but can heave or expand when subjected to moisture. This phenomenon is only due to a loose carpet. In most cases the carpet will relax and go back to its original shape – ripples and all. This is a good indication that your carpets should definitely be professionally stretched. Be sure to call a reputable carpet retailer and book in an estimate on stretching repair. This is very important if you want to get many years out of your carpet.
PS: Be sure to ask them to use a power stretcher whenever possible.