A duplex link needs the transmit fiber at one end to arrive at the receive port at the other, so somewhere along the path the pair must cross over. In duplex cabling this is called A-to-B polarity. For MPO trunks and cassettes, cabling standards define methods A, B and C that combine trunk type, adapter key orientation and jumper type to keep every lane straight.
Both ends show light out but nothing in, polarity's backwards, roll one of the jumpers at the switch.
Pick one MPO polarity method for the whole site and write it on the panel labels, because mixing methods is the classic cause of dead parallel optics.
Related terms
Roll
To swap the two fibers of a duplex pair so transmit and receive line up with the far end.
MPO
Multi-fiber Push-On: a single connector that terminates 8, 12, 16 or 24 fibers in one rectangular ferrule.
Zipcord
Duplex fiber cable made of two simplex cords joined side by side, like lamp cord, used mainly for patch cords.
Jumper
A short fiber cable with connectors on both ends, used to patch equipment to panels or panels to each other.