Replace it!!!!!
Also, check if your trunk itself seems to be even with the car all around. I know I miss aligned my trunk when I slammed it on some ice.
If you want to check where the problem is: Put something on the weather stripping like some liquid or jell. Put it on lightly, then close the trunk. Let it wit for like 5 mins. then open the trunk and see where the substance didnt touch the trunk.
