There are a few reasons that might cause the pastry to get soggy: 1. You didn't let the spinach on a sieve long enough to drain the excess water. 2. The spinach filling was still hot when you placed it on salmon fillets. 3. You didn't cut slits on the puff pastry to allow steam to escape. 4. The oven wasn't preheated enough before starting to bake the pastries.