Internet authentication problems?

Okay so my mom has a Wi-Fi hot spot on her Iphone however when she tries to connect it to her tablet half the time it says can't connect… Authentication problem. I've made it forget the network and put the password back in and what I know to do isn't working. What does this mean and how can I fix it?

Added (1). It is a acer tablet… The part I don't get is sometimes it will connect just fine then other times it wants to be picky? So I don't know…

Which tablet does your mom have anyway? IPhones don't usually connect to tablets and stuff unless they are from Apple. And even if it is from Apple but still its not connecting, then you can try jailbreaking it though its not a very good choice, but try connecting it to a Mac by a wired connection, because Wi-Fi tethering usually requires jailbreaking(or rooting for android). But still if it doesn't connect, I don't think its supposed to.

Best of Luck though.