Welcome to TFP!!
The trouble with the sunshelves are the same as those presented by steps - trying to get a liner to perfectly match the 90* angles is tough to perfectly manufacture, so many times you will end up with small wrinkles at the edges. If the steps or ledge wasn't installed perfectly level and the risers all being uniform, it exacerbates the problem. A warm, sunny day makes installation MUCH easier with less chance of wrinkles.
I hope this helps you somewhat - if you need more info from me, I'll tell you what I know (I've installed well over 1000 liners) :-D
