Comment 3 for bug 1672166

If you see any suggestions at all, then AT-SPI is available/accessibility and dbus daemon are working.

I would look at the debug output first:
$ onboard -dinfo

Type something into another terminal, then press enter or make it lose focus. You should see something like:
Onboard.WPEngine learn_text: tokens=[['<bot:txt>', 'hello']]

Note that gnome-terminal has the most reliable AT-SPI support. Others may fail in various ways, learning could be prevented too.

Also, perhaps check your language models. Onboard ought to complain if there are any errors, but just in case run:
$ /usr/share/onboard/tools/checkmodels
They all should print "No errors"