What do you use to fertilize the impatiens ? It may be too high in nitrogen. Other than that, maybe excessive shade (or on the other hand, very hot afternoon sun), moisture which varies too much, e.g. instead of slightly moist soil, the soil goes through excessive wet/dry cycles.

They like bright light which is not direct hot sun. dappled shade, morning sun or fluorescent grow lights.
Temperatures should be above 70F consistently for flowering to take place. They should be fertilized to have flower, but too much nitrogen can cause buds not to show or open. Which type of fertilizer have you been using ? how often has you been fertilizing ? Does your potting mix have fertilizer mixed in ?

Im having the same trouble with mine Impatiens also. So Whats the best fertilzer to use so blooms can occur?

To get good blooms you need to use a fertilizer that is higher in phosphorous than nitrogen. So when you look at the bag the middle number is higher than the first. Ex. 10-15-10. You can also buy fertilizer that says right on the bag that it is good for flowers.

Hold back using fertilizer. There is no need to feed what cant be fed. If the soil you are using is fresh potting soil, there is no need to feed plants for at least a month or so. And do see to the plants' drainage.

For the balcony in that north-east exposure, I'm inclined to believe its just too cool at this time of season. Why not try growing those same plants ..in a different exposure...give the container to a friend over in south-west exposure.....see what happens.

If it does result in flowering, you will know your northern exposure is not ideal for impatiens. Perhaps you could then choose a more friendly shade lover.

I am encountering similar problems. One of my flower buds was going to bloom as I could see the reddish colour of the flower. Unfortunately, it dropped off. I believe it is due to the over-watering. Try to avoid having water on the buds as well.