Hello, I have been having this problem for about a year. I also use a dongle usb and I was never able to make it work. Detail, in addition to the Zorin OS I use Windows dual boot on the same PC, and on windows everything works ok. I don't know what else to do, I also want help!
There are a few requirements in getting Bluetooth working. But the main one that is the most important, is that the bluetooth dongle supports Linux, thats your starting point.
The next thing you gotta do, is make sure your Linux OS, has all necessary Bluetooth Libraries. Yes, Windows comes with all that stuff by default usually.
But many Linux OS's are trimmed down versions, which have missing libraries. Those must all be installed. The other thing, is its best to have Kernal version 4.9 or better.
And if your running on Zorin OS 15.3, which is based off of Ubuntu 18.04, you should be running kernal version 4.9 at the very least already! And the kernal can always be upgraded to a higher version as well.
Once the requirements are met, the dongles should work perfectly. The link that I provided to the dongle that supports Linux, descusses these things in the description of the product.
In case the link dies some time in the future, here is the contents carbon copied (well, fixed a few minor typos), thanks to Sujeet Buddiga, the original author. Note that in 3) and on, you will have to use the proper string (brcm/xxx.hcd) you found in 2) to get the right result.
Run the following command on the terminal:
dmesg | grep brcm
If this is a broadcom driver issue. it will show something like: brcm/BCM43142A0–0a5c-21d7.hcd not found.