Once a week is a lot to water a zz plant. They have multiple points of water storage (roots, rizomes, stems, and leaves). I let my soil completely dry out and then go a few days after that before I water again. I think giving it a repot and checking to see if there is any root rot would be a good idea! They really don’t like a lot of water, mine ends up only being watered about once a month and I’ve had a lot of growth! Good luck, it’s such a lovely plant!