According to the knowledge base, there are two relevant points:
1. Synology released a fix for a problem where “the notes might not be synced properly between devices.”
2. If you are using DSM 7.0 or above, Note Station 2.6.0-2413 notes that it is compatible with DSM 7.0, but also that Note Station no longer supports sharing notes via Facebook, Google+, Twitter, Plurk, and e-mail on DSM 7.0 and above.
What the knowledge base does not provide is a step-by-step fix for current syncing issues between devices beyond that release-note information. So the only confirmed answer here is to make sure you are on a version that includes the sync fix and, if relevant, be aware of the DSM 7.0 sharing limitations.
Sources used: [2]