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A small dab of grease was added to ball bearings on each side and the lead screw was rotated to spread it throughout the groove. No change in the slit motion was assessed. Each slit is held onto the lead screw via compression on a fixed component moving along a linear slide. We added grease along the rail portion of the linear slide and still assessed no change.
SOLUTION: After exhausting all potential points of friction, we stepped back to assess rotation feel. The resistance felt as though the lead screw was difficult to rotate with grease, so our final theory was that the lead screw was held too tightly within the square inner chamber. We used two clean wrenches (one held at each of the positions noted in photo below, part of the lead screw is actually flat to put a wrench on it) and loosened the nut (only 1/8 of a turn at a tiny–extremely minute movements and checking movement as going) holding the lead screw into place. This ended up eliminating the friction we felt and allowed the slit to move with comparable motion to the other three slits.
Closed up and pumped back down.
ODD NOTE: Although we did not mess with the controls, we had encoder issues after working with the slits. Take note that this might be possible.