Too much nitrogen fertilizer can do this, especially if it is high in ammonium nitrate (which most fertilizers will be, since they are all based on urea).

I recommend looking into a bottle of 0-10-10 (available at homeharvest.com for example). Use it instead of fertilizer for the moment. It adds no nitrogen but phosphorus / potassium.

another option is just to hold back on fertilizing now.

Too much water, nitrogen and shade will make impatiens flower poorly.

Impatiens are really light feeders.

Yes. When I stopped fertilizing mine, they got a lot more flowers and a lot less foliage. Now IF I fertilize, I use a bloom-encouraging formula so they don't get too much nitrogen.

I have found the same.....too much fertilizer...will produce lots of green but fewer blooms.