Lemon squares can turn out runny for a few reasons. First, there could have been an insufficient cooking time. If the lemon bars are not baked for the right amount of time, the filling may not set properly resulting in a runny texture. Second, if there is too much lemon juice the filling may be too runny. Third, the lemon squares may not have cooled for long enough to be able to set. If you try to cut into them while they are still warm or not fully set, the filling may appear runny. And last, using low fat dairy products (if you are using coconut milk) can contribute to a runny filling. Ensure that you're using full-fat ingredients for the best results.