ADS-B --- Live traffic display in MP intermittently

I installed the uAvionix ADS-B PingRx recently to my Quad running Arducopter V4.0.5 on Pixhawk 2.8.4. PingRx is connected to serial 4/5 port of Pixhawk. Live traffic is displayed on MP someday, but the next day, there is no live traffic (compared to online flight tracking software and local airport traffic movement), even though the PingRx LED flashes.
I also tried connecting it to Serial 1 and Serial 2 port of Pixhawk on the day when no live traffic, but nothing change. I did not change any setting before seeing no live traffic after last live traffic display.
Appreciate any help.

I attached the screen shot of the ADS_B setting. The SR1_ADS_B and SR2_ADS_B are both 5 Hz.
Thank you.