The eyelashes that I have done before have the eyelashes fall off within a few days. Why is this?

On the one hand, it may be the eyelash glue or grafting technique of the eyelashes you made before. On the other hand, we have 3-5 natural eyelashes that fall off normally every day, but because the eyelashes are relatively small, you may ignore the lost eyelashes. The false eyelashes are pasted on your long eyelashes when grafting. 

Therefore, when your eyelashes fall off, the false eyelashes grafted on them will also fall off. It is a normal phenomenon. After the eyelash extensions, your attention to the eyelashes will increase. The false eyelashes are thicker and darker than the natural eyelashes, so you will often notice the eyelashes that fall off after the grafting and mistakenly think that the eyelashes are falling more than before.