The need for infills has nothing to do with the quality of the eyelash extensions, the glue used to attach them, or the therapist who did the attaching. It is all down to nature.
Eyelash extensions are attached to your natural lashes. As explained in ‘Your natural lashes’, the natural lifespan of every one of your eyelashes is approximately three months. Each eyelash grows for approximately 30 days and then stays unchanging for the remainder of its life. Eventually it will fall out naturally and its place will be taken by a new one, which will grow and go through the same process. (Having an extension attached does not speed up lash loss.) The long adult stage is when the lash can have an extension attached to it.
When your original treatment was carried out, the therapist may have attached lashes to all of your lashes which were long enough to take them at the time. But when the eyelash inevitably falls out as part of this natural cycle, the eyelash extension attached to it is lost as well.
Eventually, gaps will show and, if you wish to maintain the full effect of having extension eyelashes, you need to have infills every 2-3 weeks.
So, if some fall out, what is there left to attach new lashes to? The answer fortunately is that new lashes have gown in the gaps and reached adulthood. They can now support lash extensions. ‘Infills’ involves adding extension lashes to the natural lashes which were too young to support them on the previous occasion, but which are mature enough now.
You will start to see gaps due to this natural process of shedding lashes within 2 to 3 weeks after your initial treatment. You should book an appointment for infills for that time. And you should schedule further infill appointments at 3 weekly intervals thereafter, to keep the overall extensions looking fresh.
You can help your new lashes along by using a lash tonic to promote fresh growth, and a lash conditioner to improve the condition and texture of your lashes.