I'm using PiAware image 3.5.0 on a Pi Zero.
The /boot/ssh file behavior is indeed as Milliways said. To get your credentials to survive a reboot, use piaware-config to set its wireless-ssid and wireless-password entries, or use nano to edit /boot/piaware-config.txt. On my system, the file /etc/wpa_supplicant/wpa-roam.conf (not wpa_supplicant.conf) is regenerated at boot time with the network settings in piaware-config.txt.
Check the wpa-roam.conf file. If it has the correct credentials, for some reason it's probably not finding the SSID. It only looks on the 2.4G frequencies, not on 5G. If the credentials are wrong, you can change them in wpa-roam.conf and test your changes using "ifdown -v wlan0" to make sure wlan0 is down, and then "ifup -v wlan0" to try creating the connection. The -v will give you verbose output, which may help you troubleshoot the problem. Once you get it working, copy the working SSID and psk settings to the piaware-config.txt file so you don't lose them when wpa-roam.conf is overwritten at the next reboot.
Hope this helps.