I have the same issue with my Rane mixer sometimes. What usually fixes it, is closing the Serato app and reopening. If that doesn't work, you should get in touch with Serato's actual support and not their forum if you haven't already.
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FollowDJM-S9 Not connecting correctly to Serato DJ
Please Help! Yesterday I did updates to Serato DJ, updating to 1.9.2. I also updated firmware for the DJM-S9 and updated firmware for my CDJ-2000s nexus. Now when I connect my S9, in Serato DJ it recognizes that the S9 is there, tries to connect, then I get a failed to connect error.
I am running on a Mac, 10.9.5. Before yesterday the mixer was working.
Things I have tried and didn't fix the issue include. Using different cables, restarting in different orders many many times, went to older firmware on the S9, tried older versions of Serato DJ, reinstalling firmware. I hooked up my DDJ-SR controller and that connects to Serato DJ fine.
It shows in the utility that the mixer is connected to my computer but just won't fully connect to Serato DJ. Please help! Thank you!
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@Steve > If there is a "failed to connect" error, be sure that the S9 is connecting to the computer properly by opening the DJM-S9 Utility. If you see this, your mixer is not connected:

It looks like you've tried basic troubleshooting, have you tested it with another computer?