Thanks for the promopt reply. To clairfy, this didn’t happen when I updated to H9 Control, my first version of H9 Control was the latest version for Yosemite, as my H9 is only a month or two old. This was working the first couple of weeks and now it’s not, with no changes to anything! 

The H9 is the same distance away from my iMac as it always has been (and by the way, my iMac is not that old either, late 2013.) I can connect to the H9 from my iPhone 6, so it’s not the H9 hardware/bluetooth, and I can connect to my bluetooth keyboard and trackpad on my iMac, so it’s not the iMac’s bluetooth. That’s why I’m baffled!

The only thing I haven’t tried is the system reset on the H9 as you’ve suggested, so that’s what I’ll try next as soon as I get back to the studio.

Thanks for your help thus far.