You just reminded me why I traded of my 8 row 900 for a 12 row 950.   Markers are a headache.   I finally gave up and put in sequencing valve that alternated between the markers.   One thing is that the tractor hydraulics need to be in float or they aren't going down, but they are well known for having wiring problems too.   And most likely it's the ground and good luck, you'll need it.    When you get tired of looking, put on the sequencing valve, it's worth it.   Trust me.