What spray adhesive did you use? 3M77 is widely sold
, but it can't handle the temperature extremes of a parked vehicle. You may have to look around or shop online, but 3M90 is the product the manufacturer recommends for this application.
You can also go to an auto parts store and ask for headliner adhesive. It will probable be a paint-on contact cement, so a little messier to work with.
Also consider the carpet itself. If it has a heavy backing, that could be part of the problem. There are carpet-like materials intended for headliner use without a backing, so they are very light
. I've seen all kinds of other things used to cover that strip: plastic backsplash, woven grass mats, shiny Reflectix, thin wood paneling- anything lightweight and flexible.