A new Lionel (O gage) box car would only connect with my engine with a hard impact (when the engine coupler was closed and the box car coupler open). If done the other way, with engine coupler open and box car coupler closed, they would lock normally with light impact.
The reason that a hard impact was needed, turned out to be that the pivoting arm on the box car coupling was too long and it wouldn't rotate enough in the engine coupler to lock the coupling closed.
Solution: With a dremel grinder or file, a little was taken off the end of the coupling arm and the edges were rounded nicely. It now connects perfectly, even with light impact. A dab of black paint where the material was removed finished the job.
It is suggested to carefully compare a problem coupling with one that works well to see the difference before making changes.