Interesting. Frost? Maybe call or email the garden guy (Alan) at Carroll Gardens. Maybe too much sun.

I always notice lots of bees buzzing around my nandina. Maybe it is the lack of pollinators near your parents' house? I doubt that frost would affect the fruiting since they don't bloom until early summer and the fruits don't start setting until mid or late summer.

Are they getting enough sun? I noticed that the nandinas I have in shade and partshade don't flower and berry nearly as well as the ones in full sun.

Good ideas, but some clumps are in sun, some shade. There are plenty of pollinators because there's also a miniature orchard on the property. I even purchased a different plant in case they needed cross pollination. I hadn't considered frost; but I would think that would kill the buds so there wouldn't be any flowers. These bloom beautifully, then they drop petals without forming berries. Very disheartening!

As a landscape designer, I prefer Nandina without berries. The standard Nandina are now an invasive species, as well as the berries being poisonous to some birds. https://ar.audubon.org/news/nandina-berries-kill-birds