Test the room with the window. Put a coin on a sacrifice stencil that is exposed to window light every day. When you've had enough, wash out the stencil and you will see how many days it takes to change your stencil. Perhaps your window doesn't let enough UV-A light into the room to expose your stencils
Many screen makers have covered their windows with UV stopping films like Rubylith or Amberlith.
Perhaps a curtain would help, or a cloth over the screens.