Continuously logged out?

I'm wondering if anyone is having the same issue and whether it can be resolved.

I view the site regularly on an iPad, each time I close my iPad (without logging out) I have to log back in to CM - even when I click 'stay signed in'.
Similarly, when I am signed in and I open a new tab to view a thread, CM signs me out in that new tab and treats me as a visitor but I am still signed in on my original CM tab.

Any help or advice, greatly received.

 

I've checked and it all seems ok.
I have tried something out, the problem appears to correct itself when I'm not in 'in private browsing' so it looks like my own silly fault.
Thankyou.