I'm running 14.2 with XFCE, not kde/gnome. Bluetooth on the Nexus 9 is iffy at best - I am getting 'File being uploaded' but nothing arrives. Is Bluetooth ever reliable for file transfer and what are you using?

Bluetooth has always been troublesome for me. I use wifi instead, in combination with a great file manager for Android called X-plore, by Lonely Cat Games. It gives you several alternatives - SSH, WiFi Direct, Send Anywhere, FTP and LAN. I just use SSH. Easy to set up, never fails me, and I'm sure the speed vastly exceeds what Bluetooth provides anyway.

I suppose you could set up a SSH server on your phone too, with something like Termux perhaps, if you wanted to initiate the transfer the other way round (from computer to phone), but I'm happy enough initiating from phone. Transfer is both ways anyway.

Did you check your Downloads folder? That is where all my files come via BT.

Yes. I have had files in Downloads, and in a subdir of the ~/bluetooth directory. My latest effort landed some of the first fille in the root folder. There's no SDCard in a Nexus 9 and OTG storage is also impossible to use. The Nexus seems so paranoid about anything writing to memory that no file transfer works. I have to upload to Google Drive.

OTG is impossible on the Nexus 9? I can do it on my Nexus 6 (maybe it was added after the N9 came out)?

But I do know that file transfers using adb work (again on my Nexus 6). I've used adb pull and adb push quite frequently to transfer files over USB. But I haven't tried transferring via Bluetooth in a long time. It just seemed like it was too much effort and the other methods were quicker.