Sounds like some sort of caching going on. You cannot use any form of html caching on a vBulletin installation, it will break stuff. You will need to speak to your host if this is the case.
You can try this to see if it 'fixes' it, but it's not a long term solution, any html caching needs to be off at server-side:
AdminCP > Settings > Options > Cookies and HTTP Header Options > Add No-Cache HTTP Headers > Set to YES
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