Odd. I just did this on my car this past summer, and both slid right on. I removed the strip from the car, slid the old ones out the back, slid the new ones in from the back. There is a tar like substance in the groove at the front to keep it from going too far forward, but the seal can't move fore or aft once the strip is put back on the car. The door frame keeps it from sliding.
There is a left and a right, and there is a difference for Coupe versus Targa (at least for my '87).
Incidentally, that P/N 901.531.933.20 you specify is not for the seal you have a picture of. That P/N is for the seal that glues to the metal strip and sits under the front, triangular window (item #11 in these images):
In your pic it looks like you have a Coupe. You'd want P/N 911 531 931 02 for the Left and 911 531 932 02 for the Right.
Perhaps this is your problem? Although 901 531 933 20 is only about 6 inches long, so I think you'd know.