While the bananas may be getting many hours of light, the intensity of light during winter is much lower, so overall your plants may still not be getting enough light energy.
I live in Kansas and grow in a greenhouse. At the winter solstice (when I get the least light during the year), the daily light energy per square foot that I receive is only about 1/4 that which I receive during the summer solstice. Roughly speaking, there are half as many hours of light and the light is only half as intense, which equates to around 1/4 as much light energy.
Last winter I compensated for the lower winter light levels by building large external reflectors on the ground on the south side of my greenhouse that bounced a lot of extra light in (~50% increase in total light hitting the greenhouse). Basically my greenhouse was a combination of a greenhouse and a solar furnace. Under those conditions my bananas grew like crazy at this time of year.
This winter I'm experimenting with not using the reflectors. The results? Almost zero growth, some yellowing leaves, and the rotting of 2 pups that were recently acquired and placed in those conditions. My thinking is that there's an important threshold of minimum average daily light energy for growth and I'm failing to meet it.
Best of luck up there. I'll be rooting for you
