You will always have some clients that no matter how you advise them they will have poor retention on their lash sets. There are many different reasons why extensions don't last but bear in mind that as long as it's not occurring frequently among all your clients it will be down to aftercare or external influences out of your control.
The most common reason I find with my clients that have poor retention is simply that they do not follow the homecare. I have a client that books in a 3 week infill and to look at her face on you would think her lashes look great...but as soon as she lays down I can see layers of mascara and old makeup caking her lashes. I ask her if she's been cleaning them like it says in her aftercare and she actually admits that she hasn't touched them since they were put on. As I brush through and cleanse them the vast majority just fall off and she ends up having to have a new set.
No matter how many times I tell her to clean them she just doesn't do it and her retention is awful. If you notice a lot of makeup (especially eye liner and mascara) on a client with poor retention that will most likely be the cause. Oil build up not only from makeup but from natural skin secretions will be what is breaking down those bonds. So really drum it into them that they have to clean their lashes.
Other reasons may include the seasonal lash shed that we experience. This can vary from person to person but In the autumn and spring our lashes go through a lash shed so your clients may find that they lose their lashes quicker at these times of year.
Clients that have quite an active, outdoors lifestyle tend not to have the best retention as their lashes take a bit of a battering when it comes to exposure to the elements. Clients that like to sleep with their face buried in the pillow will also find that their lashes suffer or they will lose lashes from one side only.
If they use oily products or face creams these will cause poor retention. Contact lens wearers may also be using eye drops without realising that many contain oil.
If they experience seasonal allergies like hayfever their eyes will water a lot and they will more than likely be rubbing their eyes so it's best to avoid lash extensions at these times of year. Not only will they experience poor retention but there is a chance that they could become sensitised to the adhesive.
Humidity can play havoc with your retention so invest in a hygrometer. You will need to know the best working conditions for your adhesive and have ways of controlling it. If the humidity suddenly sky rockets one day you will need a dehumidifier and if it plummets, a humidifier (or kettle!).
Check your adhesive! If it's not a one off client then it could be that your adhesive is past it's best. Check with your supplier to find out how often to replace your adhesive.
Adapt your dipping/ placing technique. You may not be picking up enough adhesive in which case you want a swift movement in and out to pick up a little bead that doesn't set before you get to the lash. Also avoid shimmying the synthetic lash on top of the base lash, instead just place on top. When you shimmy the lash the adhesive is thinned out and also dries faster.
Ensure that the lashes that you are applying are appropriate. If the lashes are too heavy you will find that they come off sooner as well as causing damage to the natural lashes.