Q: Mac Pro 2012 bluetooth not connecting?

Was the BT device previously paired to another device?

You first have to unpair it from another device. This is especially try if the other device is on and within range,

I upgraded the internal Wifi/Bluetooth modules in my classic Mac Pro 2010 and a while ago and after that had a similar problem. In my case the issue was the as I had re-fitted the new card back and put everything else back together the antenna lead for the Bluetooth connection popped off the connector on the board. This is a notoriously common issue when upgrading older Mac minis which use the same connectors and are more often taken apart in a way that disturbs the cables. It is less common with classic Mac Pros as fewer people would do anything in the area the cables are connected.

In my case I had exactly the same symptoms as you describe the Bluetooth module was detected but no devices were detected.

Have a look at the following article and figures 3 and 4. Even if you have not upgraded your own Mac they will show the locations of the antenna connections so you can check yours. See http://www.macvidcards.com/wifi-and-bluetooth-card-installation-in-a-41-or-51.ht ml

With the original Apple Bluetooth module it is located in the location highlighted in figure 4, in my case after replacing it with a new more advanced module the cable has to be connected to a new combined WiFi/Bluetooth module that replaces the WiFi module shown in figures 3 & 5. This new module has four connections three of which are for WiFi and one is for Bluetooth and in my case the Bluetooth one had 'fallen off'. As per figure 1 you have to first remove the CPU tray to get at this.

Got a USB-3 card installed? the cables from that radiate in the same range as Bluetooth, especially if they are the "lowest bidder" cables that ship with devices.