The manufacturer of our zips, YKK, intentionally makes the zip slider from a soft alloy. This is so the slider (which is relatively easy to replace) wears out before the zip teeth (which are very difficult to replace). So, when zips are used extensively and in dusty or dirty environments the inner parts of the slider are worn away to a point where they cannot bring the teeth of the zip together effectively.
To get the worn sliders working until they can be replaced you can use a pair of pliers to gently squeeze the sides of the slider together. This should allow the slider to bring the zip teeth together. Don't use too much force because you can jam the slider.
Always try to keep the zip teeth as clean as possible. Wash them with fresh, clean water and lubricate with a silicon lubricant suitable for use on fabrics. This stops the dirt and dust from wearing away the insides of the sliders.