Hello,
This is really tough to say to be honest, because it is a random problem, it sounds like there could be an issue with your website or perhaps your database is corrupted.
Normally it would be all one or the other. All entries save or no entries save. This could even be a user error. You might just try disabling all plugins and re-enabling them. Perhaps something is interfering with the database write?
Thanks
Joseph
VFBPro Support