This is the most important thing when it comes to lashing. If your isolation is on point your clients extensions are going to be able to grow out nicely and more importantly, with no damage. There's no point posting that amazing retention picture when the lashes are all glues together.
When lashes are properly isolated it means that all lash extensions, whether classic or volume, are only glues onto 1 natural lash. When extensions are not isolated, damage is inevitable. It's going to happen at some point. Most of your clients will be long term lash clients, they want to be wearing and having their lashes infilled for as long as possible and that can be done, as long as there is proper isolation.
If the extensions are all stuck together it doesn't allow for the natural lashes to shed. If it's also glues to others it will cause all those others to be pulled along with it. As well as the extra glue weighing those little natural lashes down.
It's actually really common with newly trained lash techs. You normally start with a slow drying glue and sometimes it ends up being a little too slow., which will give you stickies no doubt about it. Make sure your quick enough for how you work. If isolating isn't your strong point practise! You can practise by applying lashes to a strip lash.
Have you ever gone to infill a clients lashes, seen extensions that are stuck together and thought 'arghhh' I'm sure they were isolated. Don't panic sometimes as gross as it sounds it's usually down to natural oils sticking those lashes together. A good lash shampoo will fluff those lashes back up.