If I understand correctly, it sounds like you have 1277A positioned too far forward. 1277A is the Lexan rear window, right? If I remember right, I had a bit of hard time bending the flat Lexan sheet and getting it positioned correctly.
I've since replaced my Lexan rear window with the new Plexiglas one because the Lexan crazed and just didn't look good.
Port Orange, Fl
RV-12 N121TK ELSA #120845; first flight 06/10/2015
RV-12 N918EN ELSA Eagles Nest Project; ready for engine & avionics