For some time one of my slide seals would not flip when the slide was opened. So, I have been manually flipping it or just forgetting about it. Finally decided it was time to fix the problem and bought a 3 ft length of rubber molding, 1 inch wide x 1/4 inch tall. A single 3 inch piece attached to the side wall is enough to start the seal at the top and then the rest of the seal flips as the slide continues to move out. Here is a pic.
I've still got plenty of molding if the other seals become an issue.