This all started when I was almost finished these cars. I had mounted them on their trucks and gave them a push on the layout, where they promptly flung themselves to the ballast. Repeatedly!
A little investigation revealed that the bolster cap was resting on the truck side frame and that there was a gap between the bolster base and the truck bolster. I created a washer from 0.060" styrene, which was what was required to create a sliver of clearance between the sideframe and the bolster. From there I had to shim the draft gear down by an equal amount to get the coupler to the right height. Not an elegant solution but it worked.
Once the client received the finished cars, he did a little research and discovered that the bolster shape in the kit was fundamentally wrong. It should have curved upwards towards the floor to create space for the sideframe and allow the car to sit over it's trucks.
We had a conversation about this and I offered to alter the cars to get them to sit right.
The change required that the bolster cap be trimmed back and the bolster flanges shaved down to give the required clearance for the sideframes.
Not a big job, but one that ought not to have happened. There's no doubt in my mind that the kit maker was aware of the issue and did nothing to rectify it or advise his customers. Am I being fussy? No doubt. But this should be basic stuff.