Retention could be subject to dozens of things. Sure, all of them have to be considered. Still, there are several that are key to it, so important that no other tips or tricks will help if these are not followed. Here's a quick overview of some basic, but fundamental factors for retention.
Adhesives will work well at certain humidity ranges (RH 50-70% for our glues). Humidity is a very important factor for overall performance as eyelash extension adhesives do not actually "dry", but polymerize (solidify) in the presence of water. So air humidity (RH) is crucial for the glue setting and ultimately retention.
Glues also have prescribed temperature ranges (T 70-74F for our glues). It's important not only for the glue as a chemical substance (a rough example of temperature importance would be how ordinary sugar dissolves in cold and hot water), but also because temperature and humidity are correlated as there is only a certain amount of water that can "fit" into the air at a certain temperature.
Just have a look at your local weather forecast for the day. More often than not, you will notice that temperature and humidity jump up and down during the day, often even going outside ranges specified for the glue. So, if environment is not controlled the results can get way too unstable, even with the same glue, unless one masters the art of adjusting lashing technique to changing room conditions. Fortunately, there are things that can make it easier. Air conditioning, humidifiers dehumidifiers will help to stay in control of the environment, plus the hydrometer to know the readings, of course.
Glue setting time
Each glue has a setting or drying time in its specifications. It is so for a good reason. The extension must be attached at the right time to have a good bond, i.e. when the glue is still fresh. E.g. when using a 1-2 sec. glue and applying extensions at a pace good for 4-5 sec. glue, it will be a bit too dry at the time of attachment, while still sticking, but will not create a good bond and retention will be poor. As the glue sets in the presence of moisture in the air, naturally, it will set slightly sooner in a humid environment and a bit slower in a dry one. The slight difference, even within the glue "working" range, say, at its lowest and upper limits, can often be enough to notice the difference.
Like with any glue, lash extension adhesives love clean surfaces and hate dirt and oils. The science here is simple: anything in between the glue and the surface to be fixed will prevent contact, i.e. the glue just won't get to the surface attached. It's the same story with the lashes. In addition to external dust particles and make-up residue, they will have a film of natural body oil. These must be removed for the glue to grab onto the lashes really well. That's where primers come in, and we, too, have one in our line.
No matter how hard you try on your end, clients can ruin most perfect sets if ignorant of how to care about their extensions. So, clients must be kept educated about proper aftercare to enjoy lasting results and issued aftercare tips and/or kits as appropriate as a part of the service.