I find Brunnera macrophylla 'Jack Frost' to be very long lived and have numbers of plants up to ten or more years old.

It is nowhere near as vigorous as the species, which is also quite a seeder.

As they say, Brunnera does like moist, well-drained, rich soil with lots of organic matter.

Perhaps in that location the soil is now exhausted.

It's probably due to the fact it's under a tree, the soil is probably too dry, due to the tree taking much of the moisture combined with the fact it's under the drip line (I'm assuming). Brunnera (all of them, not just Jack Frost) prefer moist soil, they perform quite poorly in drier conditions.

If you really want brunnera in this location, I would suggest digging up and re-planting in a spin-out bag or digging a large hole, lining with plastic (with bottom cut out or large holes for drainage), filling with top-quality soil and replanting. Or, you could use moisture-absorbing crystals (e.g. SoilMoist) mixed into a large, freshly amended planting hole to help get the root system well-established and better able to withstand some dryness. I've used these tricks in the past, and they do work, but it is imperative you offer supplemental moisture if the plants are under the drip line.

These are the most pitiful plants I've ever tried to grow. They can't take the heat here. I'm trying a new one called 'Sea Heart' that seems to be doing much better.