From the photo you sent it looks like your impatiens are just waiting for more time and warm weather to bloom. I see buds so I predict lots of flowers in your future. I do suggest you fertilize with Osmocote Plus by mixing the fertilizer granules into the soil if you have not done so already. I am not sure why you used plastic to line the wooden window box as this may interfere with free drainage and the impatiens need a container that drains after watering. Perhaps you could use a knife to slit the bottom of the plastic and make sure the box drains. Keep growing, Marianne Binetti