Success or failure?

This is a small ceramic pot designed to provide a nesting site for Robins.
I placed it about five feet anove the ground in the centre of a Leylandii hedge a couple of years ago.
On checking it today I found that it had been used this year by Robins.
I brought it out to clean it, and found than the nest contains one unhatched Robin's egg and fragments of others.
What is mot clear is whether or not the occupants succeeded in raising a brood and so this remaining egg is simply a sterile one; or alternatively whether the nest was actually predated (possbly by one of the local cats that seem to regard our garden as theirs) and then abandoned.
The jury is out on all that.
On the brighter side we do have immature robins around the place, so perhaps the nesting pot was a successful home for them.
It is now cleaned out and relocated slightly higher in the centre of the hedge for future use.