Has your lash tech explained to you the growth cycle of your eyelashes? Everyone sheds 2-5 natural lashes everyday. This is why you have to get them filled for them to remain full. When an eyelash extension falls off, it should one single eyelash attached to it. This is totally normal, and will happen everyday. The eyelash has a fairly quick growth cycle, ranging from 45-60 days, depending on your age and health of your eyelashes.
If there's not a natural lash attached to it, the extension fell off prematurely, either from poor application or the adhesive has lost it's bonding power. Lash adhesive only provides a semi-permanent bond, lasting anywhere from 4-8 weeks, depending upon the strength of the active ingredient cyanoacrylate. If you experience several lashes falling off within the first 3 days after a fill, you need to address this with your lash tech, there is probably an issue with the adhesive.
Cyanoacrylate is finicky ingredient that demands a consistent and ideal environment, from the temperature it's kept at to the relative humidity in the air. Lash techs that buy there adhesive from places like amazon, do not know how long they've had the product, or if it's in a temperature controlled warehouse. All adhesive has a very short shelf life, rnging from 6 months to a year, depending on manufacturer's specifications. Once its open, only 30 days.
If all of the guidelines are met, from where it's purchased to how long it's being used, and you still lose lots of extensions within the first week after application, that means your lash tech has given you a very poor application. You should be able to keep at least 50% of your extensions on between your 2 or 3 week fill.
The 3 phases of an eyelash's growth cycle is anogen, catogen and telogen. Lash techs that are not wearing magnifying eye wear or do not use additional lighting above your head cannot see the new tiny anogen hairs because they do not have much pigment and are so small and fine, they are invisible to the naked eye. Unfortunately, a lot of these new lashes get accidentally glued to your extension and get pulled out prematurely. If repeated damage to the hair follicle occurs, eventually, the follicle will stop generating a hair.
The second phase is the catogen phase. These are your mid-length lashes, strong enough for extensions 1-3 mm's longer than the natural lash. Anything longer will result in the extension turning and flipping within a week's time because the lash is still growing, and the extension, being too long, is weighing it down. Depending on how long and how thick the extensions are that were applied, it can actually break your natural lash off.
The last phase, the telogen phase, is the resting phase. It has disconnected itself from the hair follicle, and can stay "resting" for up to 100 days, before the new lash growing in behind it pushes it our, or some sort of friction (like rubbing your eyes or sleeping on your face) or additional weight (mascara, heavy extensions) causes it to shed. You cannot harm these lashes because they are not connected to the follicle. They can hold the longest extensions, but pushing the limits of length and weight of the extension will cause them to shed faster and you will look like you need a fill in a week instead of 2-3 weeks. The telogen lashes are the lashes you will see shedding every day. When you go in for your refills, remember that it's the matured anogens and all the catogen lashes that are being extended. They are the short and mid-length lashes. You cannot put the longer extensions on them, they are not long enough to hold them yet. A fill simply fills in density and darkness, and within another week or two, your mid-length lashes are going to be as long as your longest lashes. Once a natural lash has grown out several mm's, the extension will be replaced during the fill. Your inner and outer corners will always experience the most premature lash loss because we tend to rub them more in those areas, along with a few throughout the lash line. Those will be replaced during your fill as well.
If you take anything from this article, take this...everyone's natural lashes are different in length and thickness. Just because you want the longest and heaviest eyelash extensions, that doesn't mean your natural lashes can support them. If your willing to allow your natural lashes to be damaged, you will not be able to wear eyelash extensions long term. And your lash professional should be discussing this with. Lash serums will add length and strength to your natural lashes, but take at least 4 weeks of daily use to see results. Discuss your options and side effects of different brands next time you're in for a fill, if this is something you're interested in.