Simple as it sounds - there are different glues for clusters and for semi-permanent lash extensions.
Clusters (aka express lashes, or weekend lashes) are multiple lashes joined together with a knot at the base. You apply them to a lash line, not an individual lash. They're designed for parties and days out, when you need a little eyes enhancement. Because they're applied to a lash line - they actually lay on more than one lash at a time. It doesn't affect natural lash growth, as long as they're removed same or next day.
Semi-permanent eyelash extensions are applied individually to each natural lash. They're supposed to 'grow' with one and last as long as a natural lash 'lives' (until the end of a growth cycle, which is about 7-8 weeks for a lash). When applied properly (one extension to one natural lash) they don't do any harm to natural lashes; they just shed with them when the time comes. When a lash is old enough to fall out - it takes an extension and a new lash starts growing. If you use a semi-permanent glue to clusters - they'll last longer, yes, BUT - they will damage natural lashes. Since clusters are applied to few lashes at a time - the natural lash growth is being interrupted. They're glued together and can't grow properly. You need to remember that the lashes you have are all at different stage of the cycle. Some are new, some in the middle of a cycle, some are just about to fall out to be replaced by new ones. Each day you lose some natural lashes, which usually goes unnoticed due to a size/thickness/colour of lashes. If they're stuck together with a semi-permanent glue - they can't grow, get damaged and are very often pulled out by other lashes they're stuck to. Not only does sticking lashes together damage natural lashes, but also the WEIGHT of clusters and a knot. Imagine sticking a tree branch to your arm.
You get the point?
With that being said - clusters should always be applied with a glue that is designed specifically for them and removed as soon as possible to avoid complications. If your client asks for long lasting effect - you should offer semi-permanent single lashes. If they need fuller effect, they should go for volume lashes (multiple lashes picked up at once, fanned out and applied to each natural lash - but that's a different technique that requires separate - volume training).
If you do single semi-permanent lashes - please feel free to browse our store; we stock lashes, glues and all accessories for lash extensions. If you have any questions or need any advice - message us via online chat, WhatsApp, our Facebook Page or email: firstname.lastname@example.org