Decreasing volume, no evidenct mechanical problem

Our Wheaton Omnispense is 4 yrs old, and pumping volume has recently been declining, up to 4 fold now. Occurs roughly equally with all 4 of our tubes, and we compensate by increasing the programmed volume, but still getting much lower actual volume. Yet everything about the pump looks and sounds normal. Can't be the tubing, certainly doesn't look/sound like the pump. We are using 3 mm interior diameter tubing, could that be it? We want to try larger diameter later.