Lemon Bar Problem!

I’ve been making lemon bars for years–this year, a huge problem! The lemon filling formed a “thick skin” on the top! When I went to cut them–not good!
What would cause this to happen??? I checked the ingredient list over twice–I am sure I didn’t forget anything!!! Help anyone???

Did you sprinkle some confectioners’ on top? That may help. Did you place a sheet of plastic wrap on top of the filling (touching the filling) until they cooled - then remove the plastic wrap?