When the zippers are new, they are a little stiff. Be sure to grip the zipper pull closely to the zipper slider – if you do this and gently guide your zippers around corners, then your bag will last A LOT longer than if you simply grab the pulls and yank!
“The most common reason zippers get stuck is that dirt and debris often lodges in the zipper slider. Cleaning and lubricating your zipper helps to extend its life and prevents sticking, oxidizing, and jamming. The procedure we describe here works for both coil zippers and plastic tooth zippers. Lubricate your zipper after you?ve been in a dusty, dirty or wet environment.”
Step-by-step instructions and photos here.