Someone else posted yesterday, they couldn't make AFT work with a Samsung device. It has been a known problem for a while now, and is something that the AFT developers definitely need to fix. Don't expect any help from Apple of course.
Usually I don't actually connect my phones to the Mac, and do most data transfers via WiFi, either using Airdroid or FTP.
AFT has never been updated ever since it came out, and is still version 1.0. It might have worked with Samsung devices originally, but Samsung may have changed something in more recent devices and their Android versions, and it's broken AFT with them.
I found with the Oppo phones I've had, AFT works perfectly with my Macbook. And a Lenovo phone and an Onda oBook tablet works with AFT.