Why have some eyelash extensions fallen out just a few days after they were professionally applied?

Each of your individual natural lashes are expected to fall out within a month, depending on your individual growth cycle.
It is impossible to know where in the cycle each lash is in. Your lashes will fall out naturally. Every day we should expect to loose up to 5 natural lashes. Unless they fall into your eye, you won’t notice the loss. Yet, when you have added synthetic eyelash extensions, you will now notice every loss. Your lashes are now darker, thicker, longer and more lush. And because you have spent time to indulge in this beauty enhancement, you are going to naturally be a bit more sensitive and aware of any lash loss.
Sleeping on your back also helps protect your lashes. Sleeping on your stomach or on your side could mean your face-and eyelashes are flat on the pillow. This will break or damage your beautiful new lashes and the growth cycle will be accelerated. Learn to sleep on your back and protect your luxurious new lashes.