A few potential issues: 1. Your frozen spinach had a lot of ice. In this case, there's not a whole lot you can do, so just continue to cook down the spinach until more liquid evaporates. 2. You haven't simmered the creamed spinach for long enough (or at a high enough heat). 3. You forgot to add in the flour. The flour and milk form a thick sauce that coats the spinach. Without the flour, the milk won't thicken up on its own.