Not enough information, but be sure you're running latest zf7731 PtP bridge firmware:
Need to know your LAN network stability or other issues on both sides, as you report
the WLAN between bridges is staying up. What kind of switches are 7731s connecting
into, what other devices on the switch, do they see LAN latency or other problems?
Collect a support info file from both side 7731s, and the port interface details from both
side switches they're connected into if you need Tech Support to assist.