Homebrew PCBs

i am having trouble exposing a 12" x 12" pos phot board. i have done 
this before on smaller boards successfully. i am using two MG 
chemicles 15 watt light fictures at each end of the board. the center 
of the boards develop ok but the corners aren't getting enough uv. 
   i was wondering if anyone has successfully exposed a 12" X 12" 
board ? if so how.  thanks