In no particular order:
a. Uneven rail heights, so some key tires don't make contact when most needed. The long frames are stiff and may be rolled just enough to lift one or more tires clear by a rail height just 0.5mm too much or too little. This could be transversely, that is across from each other, or longitudinally with dips and humps;
b. The points are weakly hinged or improperly placed such that the relied-upon contact between them and the flange face of the stock rail is intermittent or simply absent; and
c. the turnout, as an appliance, is not firmly established on planar roadbed and the weight of heavier items causes the turnout to wobble, sag, squirm....something that weakens the electrical contact back at the last joint.
I have had to fix points so that they are sharper and lie more flush to their partner rails. I have had to bend the diverging point so that gauge problems weren't encountered. I really did have to re-lay one turnout when everything else I had tried, except pulling out that last single hair, had failed. I smoothed and flattened its supporting roadbed, which I believe made it sag or roll less when my new confounding HO Lionel Challenger ran over it. That cured the problem.
I would energize the turnout where it is, and then run probes along all the routes in several places, trying to make one or both points rails wobble or shift to see if that breaks the continuity. With the probes held to the routes with one hand, carefully, press the points end joints, one at a time, to see if electrical contact is broken. Could be a weak/dirty joiner.
Also put a drop of cleaner on the rivet at hinge point on the moving rails.You might have to solder a small piece of super flexible wire around the hinge, on the outside of the rails so the wheel flanges don’t hit the solder. Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California Turned 84 in July, aging is definitely not for wimps.
Sometimes it is a contact issue- a couple of techniques that may help. On the points where they touch the rail may need cleaning. You can use some rail-zip with a Q tip or CRC contact cleaner. (Make sure to get the plastic compatible CRC)
First of all I know that this is an ancient issue but I am frustrated in finding an answer. I have a dc layout and I really would like to find a solution to this issue.
Years ago I purchased some Atlas customline code 100 #6 turnouts for my yard in a dc operation. On some of my turnouts, the locomotives come to a grinding stop when they enter the curved diverging route. What do I need to do to make this work? I am looking hopefully for a simple solution as I am not using switch machines but rather caboose ground throws. Do I need to power the frogs? If so how?