Which version of iOS are you using primarily?
Can you verify that the bad printers all have an 'M' in the part number (found on the bottom of the printer)?
The latest firmware version for QLn320 is V68.20.15Z. Could you also provide us the full output of ! U1 getvar "bluetooth"? The full output should look like below:
bluetooth.discoverable : on , Choices: on,off
bluetooth.friendly_name : XXQLJ141300691
bluetooth.version : 5.5.2
bluetooth.date : 10/03/2017
bluetooth.local_name : XXQLJ141300691
bluetooth.address : AC:3F:A4:0B:DE:1F
bluetooth.bluetooth_pin : *
bluetooth.short_address : AC3FA40BDE1F
bluetooth.radio_version : 3.0
bluetooth.enable : on , Choices: on,off
bluetooth.connected : no
bluetooth.minimum_security_mode : 1 , Choices: 1,2,3,4
bluetooth.connected_security_mode : nc
bluetooth.bonding : on , Choices: on,off
bluetooth.enable_reconnect : iOS_only , Choices: off,iOS_only
bluetooth.json_config_channel_enable : on , Choices: on,off
bluetooth.page_scan_window : 60 , Choices: 15,60
Please pay attention to the highlighted output. The Bluetooth "enable" needs to be set to ON, and the "discoverable" needs to be ON and the "connected" needs to be NO, in order for an iOS device to discover the printer. If the printer is already connected to another device, you would not be able to discover it. Hope this helps.