Eco sdp 1630

I have an ECo 1630
Uptil now has been a good machine recently i have noticed on the two outside edge of the plate the silver image is not as strong as the middle of the plate
When on the press the plate runs ok to start with then the outside edges of the image start to disappear
Not sure where to start with this
Tried the usual new chemicals new roll of film but no success
anyone had this before
Thanks