Can anyone help me with disappearing Outlook Calendar access?

I'm on a company network, and should have access to three diaries. Their owners have granted me access via Tools, Options, Delegates (I'm computer literate so have watched them and can say with confidence I know they've done it right), and I have had the confirmation email to say I can now read and write in their calendars and inboxes. Yet these permissions drop away almost immediately. I am perplexed and cross, as it's getting annoying now, and our IT helpdesk have been unable to resolve it. Is there a knowledgeable techie or geek out there who can help me?? ;)