So I am not the only one to have had a problem with this! I had my problem start out as I would unlock my phone, go to check my notifications and it would lock on me... everytime. I ended up having to go from my home screen to my left or right screen and then bring down my notifications bar. It got to be annoying. My husband called Sprint for a problem with his phone and told him how to check for his system updates and he updated my phone as well, thinking that would help, well it stopped locking on me when I would check my notifs but then it stopped locking all together. I got a bit frustrated and took it to Sprint after it was doing this for a day or two and he did a complete restore and figured that might fix it. As I was leaving, I went to check my phone and try to lock it... nothing. They ordered me a new one and it works perfectly now.
Plus it isn't as dinged up as my old one.