FortiAP devices are thin wireless access points (AP) supporting the latest Wi-Fi technologies (multi-user MIMO 802.11ac Wave 1 and Wave 2, 4x4), as well as 802.11n, 802.11AX , and the demand for plug and play deployment.
Asymmetric power issues are a typical problem.  Wireless is two-way communication; high power access points (APs) can usually transmit a long distance, however, the client’s ability to transmit is usually not equal to that of the AP and, as such, cannot return transmission if the distance is too far.

To solve an asymmetric power issue, measure the signal strength in both directions.  APs usually have enough power to transmit long distances, but sometimes battery-powered clients have a reply signal that has less power, and therefore the AP cannot detect their signal.

It is recommended that the transmission power of the AP is matched to the least powerful wireless client - around 10 decibels per milliwatt (dBm) for iPhones and 14dBm for most laptops.

Even if the signal is strong enough, other devices may be emitting radiation as well, causing interference.  To identify the difference, read the client Rx strength from the FortiGate GUI (under Monitor > WiFi Client Monitor) or CLI.

The Signal Strength/Noise value provides the received signal strength indicator (RSSI) of the wireless client.  For example, a value of -85dBm to -95dBm is equal to about 10dB levels; this is not a desirable signal strength.

Confirm the transmission (Tx) power of the controller on the AP profile (wtp-profile) and the FortiAP (iwconfig), and check the power management (auto-Tx) options.

Controller configured transmitting power – CLI:
config wireless-controller wtp-profile
  config <radio>
    (the following output is limited to power levels)
      auto-power-level : enable
      auto-power-high : 17
      auto-power-low : 10

Actual FortiAP transmitting power – CLI:

iwconfig wlan00

wlan00      IEEE 802.11ng   ESSID:"signal-check"
Mode:Master Frequency:2.412 GHz    Access Point:<MAC add>
Bit Rate:130 Mb/s   Tx-Power=28 dBm