Grab a graphite pencil and rub the pencil tip on the teeth of the zipper. Try it again and it should work. If that doesn't work, it's time to move on to a lubricant. Windex is good because it's not oil-based, but you can also use bar soap, or lip balm.
Also know, how do you fix a sticky zipper?
Close up the zipper and rub an old wax candle or a piece of wax up and down on the closed zipper. You can rub the wax on both sides of the zipper. The open and close the zipper a few times to get the wax to spread on the insides of the zipper. Then the zipper should work much better and slicker.
How do you fix a rusty zipper?
Use soap or candlewax to lubricate the zippers before you go to a salty environment (keep in mind that wax can attract dirt so you will still need to rinse or wipe your zipper off to prevent wear.) If zippers or other metal parts show signs of corrosion, clean with white vinegar and an old toothbrush.