What happened, why didn’t she nest?

The pair showed signs of nesting - they went through pair bonding, she came multiple times to modify the nest depression, they hooted back and forth identifying their territory, but then it stopped.

There are different reasons this may have happened. Great-horned owls have a reproductive life of approximately 15 years. Of those 15 years they reproduce between 5 and 8 of those years. This pair has been successful multiple years. The female may have come to the end of her productive life or they may be on an off year.

Of course there may be other reasons why this happened.

This is very disappointing to us, we (as you) were looking forward to observing all the nesting stages and watch owlets grow to fledglings. If she has reached the end of her productive life then this pair will be leaving the territory, whereby another pair may claim this prime area.