Common sympton when have interference and/or not enough Zigbee Router devices, begin to reading and try following all the best practice tips here → Zigbee networks: how to guide for avoiding interference + optimize using Zigbee Router devices (repeaters/extenders) to get a stable mesh network with best possible range and coverage
That includes a tip to upgrade the Zigbee Coordinator firmware, for Sonoff ZBDongle-P see → ITead's "Sonoff Zigbee 3.0 USB Dongle Plus" (model "ZBDongle-P") based on Texas Instruments CC2652P radio SoC/MCU