Your scissors fold the hair instead of cutting it. This is a good indication your scissors need sharpening or adjusting. Beauticians and groomers use expensive shears to cut hair. You can't just drag out your old Fiskars® from your desk drawer and start hacking away; they don't fit your hands the way real hair-cutting shears do.
You must be able to handle your shears with absolute ease. Always be in control of your shears and make sure they fit your hand and fingers snugly. Always be aware of where the tips are before you close them; a dog can jerk suddenly and you don't want those points poking you or your dog.
If you use dull shears, you are damaging the ends of the hair instead of cutting it. You may eventually cut the hair, but you are literally crushing the ends of the hairs, and that will lead to straggly ends that catch more debris and other hairs, creating matting.
Who Sharpens Shears?
You can't just trust anybody to sharpen shears. If you think you can do it yourself with an inexpensive scissor sharpener, you may as well toss out your shears. Shear sharpening isn't just grinding them down or using a sharpening stone. High-quality shears should be sharpened professionally by someone who knows what he is doing. Ask your groomer or beautician whom she uses.
To check the sharpness of scissors, get a quality paper towel and cut a slice into it. If it cuts through easily from the back of the shears all the way to the tip, your scissors are sharp. If you feel your scissors bumping along, you may have a nick or burr on the blade that your professional sharpener can remove. If it folds the towel instead of cutting it, it needs some serious help.
As with good clipper blade maintenance, you should only use your shears on clean hair. Using expensive shears on dirty hair is abusing them. There is hidden dirt, grime, and other particles you can't see that will dull a shear. Cutting dirty hair can damage the hair as well.