Sony Ereader Prs 650

I don't use the Reader software on Mac often, but the last time I used it, it worked. Now my PRS 650 won't connect to the Reader software, although both SETTING and READER devices appear on the desktop. Latest available Reader software (v2.4.01.10241, latest available firmware for PRS 650 v1.0.00.14080). IMac 27' 5K 2015 running El Capitan 10.11.6. (It's always run El Capitan, since acquisition in November 2015.) I've tried reinstalling the Reader software, reset the eReader, and plugged and unplugged the USB lead several times (directly into the computer, not a hub), using three different leads all of which work with other hardware. What's changed to stop it connecting? And what if anything can I do to fix it?

According to the Sony website, the Reader software is only compatible with Mac OS X up to v 10.9. If so, why has it worked before? Xyzware software.