Hey guys,
I had the same probelm with mine. It actually stayed in place for a month then when I had the car out at a car show and it was like 100 degrees , the velcro adheisive actually let go from the trunk, not the undercover.
I then stripped off all the adheisve from the velcro backing that attached to the truck side and used the 1" 3M body tape (Napa, AutoZone or Advance Auto). This held real well for several months, then I opened the trunk one day and one of the sides had let go.
I took everything off and have not installed it since. At this point I am frustrated and not sure what to do. It seems when it's like 90+ out and the car is in the sun, it falls off.
Any ideas or thoughts would be great! I thought I solved it with the 3M tape, but apparently not.