Once in a while, my Optimus S fails to lock onto the GPS satellites, even when I'm outdoors with a clear sky. I don't know why. (It doesn't matter which GPS-enabled app I use.) I posted about that issue a few months ago, and a couple people replied that they had the same problem. Try shutting your phone down your phone and restarting it.
Of the 4 Optimus S's that I bought for me and my family, one of them seemed really bad about locking onto the GPS satellites, so I returned it to the store within the 30-day and got another Optimus S, which seems to do better at locking onto the GPS satellites.
