How many sensors do you have? Unlike zwave, which allows devices to pick any available repeater, zigbee uses a parent/child structure where repeaters are permanently assigned in advance and each repeater will only pair up with a limited number of children, typically 3 to 5. Even the hub has a limit of 32 or 64 (depending on the hub model). So one repeating device may not be enough, regardless of where it’s located.
https://community.smartthings.com/t/faq-32-zigbee-direct-connection-device-limit/