GMNotes (and Notes and Bio fiels) are only loaded when you first open the handout or token. There might be a network issue or a slow shard that timeouts and prevents the browser from loading the notes. It happens quite frequently to me too. Closing and clicking again (after some time) should help.
Via http://status.roll20.net/ you can see that the average responsetime of the shard is momentarily around 400-500 milliseconds, so with a text with embedded pictures and some network hickups those halve seconds start to add up.
If this occurs: never, ever change the item and/or save it! It will overwrite the item in the shard/database and you lose it.