1985 Kenworth signal issue

I have a weird issue with my 1985 kenworth W900B. If I hook up to some trailers in the daytime I get signals ,hazard lights and brake lights on the trailer .if I turn on all of my running lights (for night driving ) I get everything but no signals on the trailer.The tractor has signals always no matter what .I have a trailer light cord 7 way checker and when I plug it in to the trailer side of the tractor light cord it shows everything working fine as soon as its plugged into a trailer everything works but signal lights on the trailer ...any suggestions....grrr.ripping my hair out over this .Last night I replaced the trailer light cord with new one and my signal stat switch and same problem ..the tractor signals work but not on the trailers

 

I want to thank everyone here for your replies and it was a ground issue ..happen to speak with an elder gent who has worked at Kenworth for over 30 yrs and he said to replace regular flasher with a L.E.D. flasher that comes with a ground wire to ground the flasher because I have all l.e.d. lights on my older KW and L.E.D. lights draw almost no power and act as an open circuit where incandescent lights act as a short circuit .Well I did what he said and my truck is working 100% normal again ..everything is working ..again thank you everyone for your replies and your great advice and suggestions .Never knew of an actual L.e.d flasher with ground wire existed

 

L.e.d. on tractor and l.e.d.s on trailer