Question for flatbed screen printers

We have several M&R flat bed screen presses. We print mostly on coroplast and styrene. We have an issue with all of are presses and all of our print heads when we try to print a maximum print area. Once we go beyond a certian width, we have to really over impress our squeegee presure to get the outside edges of our image to print. Or, we have to shim the hell out of the squeegee at the edges. An M&R engineer informed me I was the only print shop he had heard having an issue like this and pointed the finger at our processes here in or shop. We have a 38 x 50 Conquest, two 30 x 40 Saturns and a 40 x 56 Renegade that all print just fine with our current in house processes as long as we are printing within a certian width. Just thought I would post here and see if any one else has issues like this with the same style presses. Thanks for any input.