It is the tilting of the scales which is the big issue, at 10lb it is easy to hold them straight but when you start getting up to 30, 40 and 5o or whatever it is very difficult to keep the scales hanging straight. Once the twist a fraction and the fish maybe bounces in the sling the weight increases and does stick.
Where possible I hang the scales from a tree (making sure the branch will easily support the weight first by pulling heavier on the scales) or at the very least I use a weigh bar. As you rightly say it can be difficult gripping the scales particularly with very big fish without the scales juddering around.
Best fishes
Shaun

I’m afraid you are very wrong and simply attaching your scales to a weigh sling with let’s say 20lb (of anything) and weighing it first by the handle of your scales and then by cupping the sides of the scales you will see that you are able to get a much higher reading by cupping the sides.

If you can’t I’m afraid you will need to return your scales to the manufacturers as they will most certainly be faulty.

Hope they are okay
Best wishes
Shaun