Have you checked the configuration of the plugin? Something might have changed in that. Check that it is still set to start automatically for your device. It shouldn't change unless you do it, but, any changes could have affect. Also make sure the columns in the configuration are correct. If you changed them in some way, it could have affect here.
Otherwise, we get to the usual questions:
- Are there any errors shown? Or other messages?
- What version of calibre? OS?
- What firmware version? All the firmware versions for the Libra H2O should be OK, but, it is a detail that can be useful.
- Are the books on the device that you have been reading matched to the books in calibre? That means there will be a green tick against the book when you connect the device.
The answers should help, but, running calibre in debug mode and posting the log will probably tell me what is wrong. You can do this by right-clicking the preferences button and selecting "Restart in debug mode". Calibre will restart and display a message about the log file. Then connect the device and let the device jobs run. At the end, close calibre and the log will be shown. Post or PM the full log to look at.