First thing I would do is check to make sure that all the pins are straight and equally spaced ... then look at the plug itself and see if you can see extra holes in the plastic surrounding the metal female receptacles.
Why, you say ??? Well, because I've plugged my units in, in a hurry or from a bad angle, and actually pushed one of the pins INTO the plastic of the plug, rather than the receptacle it's intended to go in. After finding my error & pushing the pin back into alignment (on two separate occasions) it worked just fine. Well, that is until the third time it happened ... and one of the pins broke when I tried to bend it back into position. That turned a $500 Lowrance Elite 5 DSI unit into a paperweight. (unless or until I can find another one that's been junked for reasons other than the elec connection assembly, that I can buy for around $50 and swap out the pin assembly) I've already purchased a Hook 7 to replace it, but haven't mounted it on the boat as of yet.