Sorry to hear it isn't going well. It can be a lot of things, let's start with these:
1) if you plan to do this for many years, invest in a Brinsea calibrated thermometer, and a hygrometer (I can post a link to the two I've successfully used, some don't fit the incubator and others go on standby so you can't read them).
If the temperature in your incubator is 1.5C too low, lots of embryos will die throughout the 21 days, you can lose half of them that way.
Like Chris says, if the humidity is too low or too high, you are likely to loose many mainly towards the end when they can't turn to pip (too large) or couldn't grow big enough. Also, crank up the humidity for the last few days. Keep the incubator closed as soon as the first egg has pipped.
You need to know the humidity in the room the incubator is in before running it dry for the first 18 days (and having it much higher at the late stage).
In my house, heated and in the dry home counties, the ambient humidity is hardly ever good enough. Chickens don't sweat but their warmth causes humidity from the nest and ground they're sitting on.
Follow the Brinsea guidelines about temperature and humidity very precisely, it helps.
2) get your own parent stock (provided you can keep a cockerel).
Like Chris', from our own birds, both fertility and hatching rates are also over 90%. From other people's chickens' eggs, rates tend to be far lower.
3) keep the incubator away from sunshine and drafts like you already do but also in a room where the temperature is stable. Perhaps put it inside a wardrobe if need be?
4) don't panic yet, the chicks of one of my hens a.l.w.a.y.s hatch on day 22 and they're super healthy
Tomorrow will be day 23, right? Maybe candle them tonight and see which are fully developed? Knowing when they stopped developing will help you narrow down the causes. Wait until day 25 before opening any fully developed ones up to see what might have gone wrong.
Good luck, hope it goes better than expected today!