got an H&R Topper that is older than Jesus Christ himself ... it failed to eject the shell. ...
any idea as to why it might be happening?
One reason could be a rough or dirty chamber. Clean thoroughly and burnish the chamber with some 0000 steel wool wrapped around a brass bore brush driven by an electric drill.
The most likely explanation, though, is that over the last 2,000 years the ejector and ejector spring has become clogged with dirt / old grease / ancient goop.
Another possibility is that the ejector spring has lost some of its power.
There is also an ejector latch and small associated spring. Their movement could be impeded by dirt or rust.
You could try cleaning the whole thing with some brake cleaner, and then lightly oiling.
If that doesn't work then the solution will be to disassemble, clean, lubricate, possibly replace the ejector spring and reassemble. This is facilitated by the removal / installation of the pins located in the ejector housing.
I hope this helps, best of luck.
Sharptail