Headlights low setting not working with headlights switched on.

[twaddle]

I am having some trouble with my 1996 K1500 Diesel Suburbans Headlights.
When the light switch is on the marker light setting my headlights are on low setting for daytime running the way they should be. But when I turn the the headlight switch to full on (headlight low light beam)on the headlight low setting headlights will not come on. Marker lights are all on but no low headlight.
If I put the headlights onto high beam setting the high setting works ok.

I have the high beam headlight set up with the modification to bring on the low beam setting at the same time as the high bulb so I know that the low beam light bulbs are good.
The modification to allow the low and high headlight to work at the same time was done many years ago when the truck and me were still living in Scotland across the pond and has never given any trouble (composite headlights are on this truck).

If anyone has had this problem and found the cause I would be grateful if you would let me know what might be causing it before I start chasing my tail .
Thank you Jim Twaddle, Alberta, Canada