I have hatched 11 out of 12 Italian quail eggs. My first chick hatched day 17 and last day 21. The one remaining egg is still in the incubator and on day 22. When do I 'give up' on the egg? I don't want to turn the incubator off just in case. Any advice would be appreciated.
I think something must either be going on with the incubator temperatures, or humidity was wrong, or else the eggs were laid over a fairly long period, to have given you such a long space between the eggs hatching. Normally they all pop out at once on day 16, maybe day 17 if temperatures are a little too low. This may potentially be more concerning than the fact that one didn't hatch at all.
Next time, you could try setting eggs you know to be fresh, i.e, laid within less than a week of each other; using a reliable incubator thermometer as well as the inbuilt one in your own inci; and starting the incubation dry, i.e, with no water in the machine, and then topping this up on Day 15 or when the eggs show signs of pipping. Quail eggs hatch much more reliably after incubation with low humidity, because the eggs are so small and therefore the surface area is larger in relation to the contents than, for example, a chicken's egg. Also originally they would have lived in hot dry areas and have evolved to deal with low humidity. too humid and they tend to be affected and drown. I'm assuming you're in the UK, where the air is humid enough at this time of year to supply the moisture the eggs need (45%) via the ventilation system in your inci. Boost the humidity in the last 24 hours to assist hatching, but as the eggshells are so thin anyway the chicks normally release themselves very quickly.
I would give up now on the remaining egg. Crack it open to see if it was in fact fertile - this is not the best time of year to be hatching, from the point of view of fertility. In any case, even if it did now hatch, there would be a difference in development of 5+ days from the older chicks, and integration might be difficult for what would be likely to be a weaker chick. You'll have observed a lot of development in your chicks over that time.
Where did you get the eggs from? And how are the 11 chicks doing? 11 out of 12 sounds a very good hatch to me - it's normal for at least one to be infertile anyway.