I've always found mine to grow super slow. I think that's why they're so expensive, and usually small in the nursery pot.

They are slow growing but I have found they do much better with a good half a day of sun. I had one in light shade for years and it just sat there. I moved it to more sun and now it is much larger! Is yours the straight green? It can take full sun and should be much more vigorous than the yellow varieties. (My experience)

Root weevil infestations can also slow growth way down.

I have two - 'Albo-striata' and 'Aureola' - in a bed on the north side of the garage. I planted them 2 yrs ago and they've grown enough that I had to move them back this past fall as they were blocking the path. The 'Aureola' gets a little sun as that part of the bed is wider, but strangely, is slightly smaller. Perhaps 'Albo-Striata' is just a more vigorous variety? Hard to say. I do recall they didn't seem to do much and then suddenly last summer I noticed how large they were when I couldn't get past them!

Hakonechloa is noted for being slow to establish and expand. IME, all cultivars will take sun quite well as long as the soil is sufficiently moist - it doesn't care to dry out at all.

The species is the fastest to establish, 'Beni Kaze' and 'Albo-striata' somewhat slower and 'Aureola' and 'All Gold' quite slow. I planted a clump of 'Albo-striata' in a container planting last summer and by the end of the season, it had taken over the pot with a huge root system. I guess that's testament to the benefit of a loose, well-draining soil, additional fertilization and frequent watering.