Stop in for a cup of coffee
Mattax, have seen that before. It's usually because there was not enough contact cement applied to both weatherstrip and the trunk flange surface.
People will put the weatherstrip adhesive only on the rubber weatherstripping and stick it on wet . . not exactly right.
Follow the outline in Post 296160 with
@Bills65Dart
If you want to get the rubber weather strip to stick really good, apply 2 coats of Contact Cement to the weatherstrip adhesive side and 2 coats to the trunk flange. Letting each coat air dry for 10 minutes before applying the 2nd coat. After the 2nd coat on the parts let the whole thing air dry 15 minutes to flash off and tack up. Then you can stick it down next, it is quite permanent. Provided you have cleaned the surfaces with a fast dry enamel reducer to prepare them for the adhesives.
Good to have a second pair of hands to hold the glued weatherstrip 4 inches up off the trunk flange, while the next person carefully sticks it down in a straight line without stretching it. Want to get the length right doing this, a little push or pull on the weatherstrip is ok while sticking it down to get the length to come out just right. This is where the second pair of hands come in real handy.
Good Luck . . and yes we want to help you keep the water out of the trunk . . .