I have created a Google App Engine Project in Java using the Mirror API for Google Glass but when I insert a timeline card, they only appear on the timeline on Google Glass when I have unauthorised and reauthorised an app
There can sometimes be a small lag, particularly if your Glass has had weak connectivity in the recent past. You can sometimes trigger it to re-check the servers for updates by sharing something in the timeline - this will force it to contact the server and will pick up updates then.
This is not good, of course, but should only be necessary rarely to get things moving again.