It is in the mixing, must be.
I test the mix using a knife if it slops off thicken the compound up, it has to hold on the knife.
The pics of our road was all done in one go, with no problems.
I use balsa strips about the same hight as the rail , I pin them where the road width will be, allowing for side streets as I go.
Then I mix and pour mix and pour untill it`s all in.
Using a pice of wood the lengh of the street I glide it across the rails balse wood to balsa wood, this will push the excess as I go into the area to do next, have a pan of water to keep the wood clean.
so as it`s drying the top rails are just about showing.
When it`s dry I use a jewlers screwdriver at an angle against the inside rails and a set of wheels to check for contact..
I dont try for a smooth finish untill its been down for 15 mins otherwise you will thin out what you have just done and weaken it .
Hope that rant helps trolleynut
Here is a shot of the last street I did on my old L/O.
The last shot is the finished scene.