When do Notes sync?

We have our grocery list in a Note synced (via Notes -> File -> Share ->...) among two iPhones, one iPad, a Mac Mini and a MacBook Pro. This arrangement is so cool when it works: when one of us enters a need on one device it appears on the four other devices' lists shortly.

Except that sometimes it doesn't. Occasionally I'll get to the grocery store and find out that my iPhone's version of the list is stale.

What are the rules for when these Notes sync? Is there something that I can do while I'm at the store to stimulate a sync?

--Gil