There's no delicate way to say this, so let's just say it: if your brand-new left-handed shears aren't cutting properly, you're not doing anything wrong, but you are doing something you've probably done since you were seven years old, and it's time to unlearn it.
We say this with love. Also, we recognise it's a little awkward to tell a grown adult they don't quite know how to use scissors. But here we are.
The scissors aren't broken, your habit is (sort of).
Here's the thing about being a left-handed sewist: most learned to cut using right-handed scissors, because that's what was in every classroom, and every hand-me-down sewing box since the dawn of craft time. Right-handed scissors, held in a left hand, don't want to close on their own. So without ever being taught it consciously, most lefties invent a workaround: a little "push" with the thumb and fingers that forces the blades together.
It works. It also becomes completely automatic, the kind of thing you do without thinking.
So why don't proper left-handed scissors work?
Often when a left-handed sewist finally treats themselves to a gorgeous pair of tailoring shears, they are so disappointed to find that they're "blunt" or simply don't cut at all.
Left-handed shears are built so the blades close correctly for a natural left-handed grip, no forcing required. But bring your old "push" habit to a pair that was actually designed for you, and that same motion does the opposite of what it used to. Instead of closing the blades, it pushes them apart at the pivot. Fabric and thread get trapped between two blades drifting away from each other instead of meeting, and you get an incredibly maddening result: a gap between the blades, and fabric that crumples instead of cutting.
The scissors aren't faulty. Your hand is just doing the right-handed workaround it's always done without even thinking about it.
The fix
Relearning the grip is straightforward once you're aware of what's actually happening. A few tips (again, apologies for telling you, a grown adult, that you apparently don't know how to use scissors!) :
- Let go of the push. Left-handed shears want a natural squeeze and release, not a forced closing motion. Cut slowly and deliberately, without adding extra pressure at the pivot.
- Watch the blade angle. With proper left-handed shears, your top blade should sit slightly on top and guide the cut down and in, the mirror image of how right-handed shears feel for a right-handed sewist.
- Practise on scrap fabric first - it goes without saying - this is a job for an offcut, not the good stuff.
- Give it a week. You didn't learn the old habit overnight, so don't expect to unlearn it in one sitting. Most sewists get there within a handful of uses, usually followed by a small, satisfied "oh!" the first time it works properly.
The upshot
If your left-handed shears aren't behaving, it's likely not a manufacturing fault, and it's not a personal failing, it's just a transition almost every leftie who grew up on right-handed tools has to make eventually. Stick with it, and within a week you'll be wondering how you ever managed with the old workaround at all.