volte

I have 2 Ultimaker2+ machines, lately they both have been under-extruding when printing toward the back 4inches of the machine. (positive Y axis ). When the machines are printing toward the front (negative Y axis) the print quality is wonderful. I have tried: adjusting the feeder tension, temperature, speed, replaced the bowden tube, lubricated the filament with vegetable oil and increased the extrusion multiplier with no significant improvement of the print quality. I use Simplify3D application for generating the machine code. Any idea what would be causing this?