Really, there shouldn’t be gaps in your natural eyelashes, unless they’ve been created by the way you’ve treated them. Lashes are constantly growing, falling out and re-growing, and because this is all happening at different arbitrary times and places along your lash line, new growth invisibly covers the replacement areas. But if you pluck out eyelashes at a particular time, so that they all come out together, then there will be a visible gap.
Sometimes, it’s not your fault – sorry for the insulting assumption a moment ago. A gap may have come about because a bad therapist glued an extension eyelash so that it attached to two natural eyelashes. Since one will grow at a different pace from the other, the fast-growing one will pull on the other one and give you a feeling of irritation. If you are impatient, you might try to pull out the one irritating eyelash, and end up pulling out a lot of innocent ones.
A fundamental point about individual eyelash extensions is that the extensions are put on your natural lashes. So if you have gaps on your eyelids between your lashes, or groups of lashes, then in principle, there’s nothing the therapist can do about it.
However, if the gap is suitable, the therapist may be able to reduce the appearance of the gap by adding very fine eyelashes to the nearest natural lashes on either side and have them cross each other in a diagonal line, to give the appearance of lashes in that part of your eyelid. This will probably not last for more than a week or two because the lashes are not being firmly fixed along a long stretch of your natural eyelash. Instead, because the disguising lash is positioned to jut out at a diagonal angle across the gap, it will only be fixed at the base.
Still, you need to come in anyway every two to three weeks for normal infills, so it will not be a problem to keep having the gap coverage redone at the same time when necessary.