OK, when I first started using 1Password, it was pretty straightforward, there was a third party store like Dropbox or Google Drive and then you encrypted a file with your master password.
However, they've since introduced the concept of their own storage which is great. Now you can have multiple account and multiple vaults within those account and each has a master password.
So the main question is when the thing starts and prompts for a master password when you startup, which one is it asking for?
The issue is that if you still have an old primary vault on another storage system, then you can end up with multiple master passwords on different machines. For me what was happening was that this vault was generating lots of multiple master problems, so different machines were locked onto different versions. Pretty confusing. I don't use the Dropbox stuff anymore, but if you have this problem, then you want to either delete it, when you find this problem, just change the master password or find the duplicates and delete them 🙂
The way you do this is to go to Preferences/Sync and turn them all to None and you should be ok.