it would not be the platens, they rotate from head to head. It could be many things a picture or a video would help. Trying to figure out what light means without visuals is difficult.
To clarify, you took the black screen that was printing fine and moved it to head 3 and it is now depositing less ink? Were you using the same squeegee and floodbar angles? Same pressures and speed?
based on the limited amount of info here I would guess air delivery problem or head out of alignment. Getting more info will help figure it out.
pierre