XF 1.4 Members getting logged off on phones...

Any ideas on why our members are being logged out when signing in from mobile phones? They are ticking "stay logged in". We have two reports of this today. Iphone 6 is one, and I've asked what the other is but thought I'd see if anyone knows why.

Thanks @Jake Bunce ! I have been through that list, but have enlisted the help of someone else to check it as well. What doesn't make sense to me is that some users have no problems on mobile & some do. Makes me think that it is related to something they have configured on their phones or on their end rather than our end. But, I have no idea what.