Bad Distance in real test

Hi everyone!
I set up one single channel gateway with orangepi and RFM95W (868Mhz). I have used sodaqone to communicate with SF=7, duty cycle = 1% and power index=1. I placed device beside gateway, I got RSSI = -35 , but when I went away about 50 meters, it increased up to -91. From then, the lost packet happened. How could I get the bad performance like that ?
Thanks