AT&T sim card error in wireless phone. I just got a new phone and my sim won't work in the old one anymore?

Okay, I have always taken my sim card out and switched it between different phones. I have never had any problems doing this before. I just got a new Atrix 2 and now I when I put my sim card into another phone it won't do anything. it says "insert sim." however, it is my same sim card, same old phone, and yes it is in correct. I tried doing the same with another phone line under the same account and had the same issue. Did the guy at my local retail store change something so I cannot do it anymore? how do I fix this problem?

Notes: I have an Atrix 2, iphone, lg vu, lg neon and they are all ATT and have all worked fin in the past. Currently, one sim only works in the Artix2 and the other sim only works in the lg neon. The iphone wont activate in itunes or anything. I will call ATT in a few days to have them fix it but it just takes hours and doesn't always get fixed if you don't kown what is wrong before you call them!

FYI I like to change between phones when I go camping, on a trip or around water. Ya know how it is. I have always enjoyed being able to anytime I want to.

please help if you know.