In VeraCrypt main window, go to Settings > Preferences, and look for "Preserve modification timestamp of file containers" and uncheck the box. By unchecking this option, you are telling VeraCrypt to allow the timestamp of the file container to be updated to allow the software sync to recognize that the file has been updated.
Also read the following and understand the risks.
https://veracrypt.codeplex.com/wikipage?title=How%20to%20Back%20Up%20Securely
https://veracrypt.codeplex.com/wikipage?title=Plausible%20Deniability
Also read the following and understand the risks.
https://veracrypt.codeplex.com/wikipage?title=How%20to%20Back%20Up%20Securely
https://veracrypt.codeplex.com/wikipage?title=Plausible%20Deniability