Understanding is important
To have a reasonable chance of successful queen introduction you need to have some understanding of why you may get failures. This may appear to be the wrong way of approaching the subject, but I think the elimination of as many of the likely problems as possible will increase success.
Despite claims over the years, no method is 100% successful when performed over a lengthy period. Those of us who have introduced, or tried to introduce, many queens over a long time will tell you that you can have two colonies in the same condition, in the same apiary, on the same day and if you introduce a queen in the same condition into each, one will succeed and the other will fail. If they were two separate colonies belonging to two separate beekeepers one would say the method works, the other would say it doesn't.
The information on this page is relevant to a colony on full size frames, not a small colony such as a mini-nuc that often behaves differently. Some of the normal rules don't apply to them because they are small and often out of balance.
Queens could be in different states, the most likely being virgin or fertile, the latter either recently being in lay or out of lay for some time. In general, you need to have a queen that is being introduced into a colony in the state the bees are expecting, otherwise you will greatly reduce your chances. There is no point trying to introduce a virgin queen to a colony that has recently had a laying queen, unless there are queen cells. Similarly a queen that has been out of lay for some time, such as one that has been banked is very difficult to introduce into a colony unless it has been without a laying queen for some time.
I have had many successes and failures and although there are some rules that are fairly reliable, there are others that aren't. So often you can do something you have done successfully many times before and will fail, yet you can do something in desperation, or in a rush, that you think has a slim chance of working and it will.
The pheromones that are given off by queens are individual, the difference can be quickly detected by bees, so an alien queen is soon recognised. In queen introduction we are waiting for the pheromones of the previous queen to diminish and those of the introduced queen dispersed throughout the colony, so she is accepted as theirs. I suspect that at some stage there will be more to find out, but at the moment, that is what we know.
Here are some of my observations on queen introduction, not in any particular order:-