CWNP Exam CWDP-303 Topic 2 Question 39 Discussion

After deploying a 5 GHz-only WLAN infrastructure in the USA, using 20 MHz channel-widths and all 25 available channels, one of the managers of the company brings his 802.11n tablet to the office to test the Guest SSID. While testing, there were several spots where we would get poor RSSI (below -80dBM) or none at all. You checked the WLAN infrastructure and all APs are up and running and you've validated coverage after deployment. What is causing this issue?