On the rear, the fender liners are constrained by screws at each end. I would loosen all of them up, then make sure you have all plastic clips inserted (there's a bunch), and then tighten one side of the screws down (front, or back). After you get one side tightened, go to the other end of the liner and push it up while tightening the remaining screws. There's a bit of play in the fender liners holes for the screws/clips and that might help.
I think on the front there's only screws at the rear of the liner. But, you can follow the same process as for the rears, and basically push the liner all the way up and forward before tightening those rear screws back down.