iPhone 4s won't connect to Hidden WIFI

Hello,

We have 3 iPhone 4S users who are currently having trouble connecting to ONE of our Wireless Access Points. One they can connect to no problem, the other they get an error along the lines of the "SSID is incorrect/Network name does not exist". Here is the catch, my BlackBerry Z10, a Samsung Galaxy S4 and an iPhone 5 all connect no issue, along with numerous Laptops.

I have reset network settings on one of the phones, and it did nothing.

What is the cause of this?

Can anybody help?