Allen Bradley 1756-OF8/IF16 Wiring help

Hello everyone!
I am having some issues wiring 3 Proportional valves. I am connecting the valve position reference to a 1756-OF8 AB card in a ControlLogix L55 and using a 0-10vdc analog signal. The valve position feedback is also a 0-10vdc analog signal going to a 1756-IF16.
My confusion is that the valve uses a common negative(-) connection for both analog Input and Output. I wired the valve's position Reference(+) to VOUT and the valves(-) to the RTN on the 1756-OF8. The valve's position Feedback I wired to the IN(+) on the 1756-IF16. I was thinking both cards RTNs are internal connected.

So when I gave #1 valve a 60 degree position reference from the PLC, the valve moved to that position and I received the corresponding feedback signal. Now this is where I get confused... When I gave #2 valve a 90 degree position reference, the #1 valve's position reference went from 60 degrees to 45 degrees and #2 valve only opened to 45 degrees. Giving the #3 valve a 90 degree reference all the valves moved to 30 degrees. It appears that the voltage signals are being divided?!?

Can anyone help me out here?

Thanks,
Shane

I don't think it's a safe assumption that the returns are common between the two cards.

You are correct sir! They are not common from card to card.