It sounds like your latches need lubricating. Try a spray lubricant. But I warn you. There are a few lubricating sprays that react badly with certain plastics, melting the plastic. Then you'll need to replace the latch. Test your lubricant by spraying some on a rag, then rub rub rub on the outside of one plastic latch. If it gets tacky, DON'T use that lubricant. I recall some silicone sprays can be bad on some plastics. It's not the silicone itself, but rather the evaporating carrier liquid that does the damage.
It is also possible that your latches need adjusting.....maybe both lubricating and adjusting.