My 1995 soft top, having been left down for a very long time, has shrunk so it won't go far enough forward to allow the top's cam to reach the catch on the windshield frame. The fabric seems like the original Sonnenland canvas. I read that this is a common problem among many makes and steam seems to be the solution followed by cooking in the sun once locked and stretched. I doubt there are any specific Porsche tricks but I ask because some other marques have an bolt adjustment mechanism.
there is a workshop manual for the top itself, in which it describes the adjustment for the reach limit.
Further explanation on my cab top that doesn't reach the windshield and thus can't lock down. The top was a replacement - it has shrunk and one catch was sheared off by PO who tried too hard to crank it shut and locked. I was lucky to find on Ebay a pair of latch brackets (they come left and right). Top still didn’t reach windshield well so got another 1/8'' length by filing down a bolt head. Still requires some weight on the top to push it the last 1/8 to lock securely - but it does. Not a concern because it’s a sunny day driver and the top is only up once or twice a year - stored winters with top up and locked. Tried steam plus hot sun on wet roof didn't work, Tried bolt adjustment - didn't work for me but not sure I did it correctly.