May be a network issue. Try reactivating your phone and hard reset it. *228 and prompt #1. Then reset your phone by taking out the battery waiting a few minutes and put it back in. I had the LG chocolate touch as well and it did same thing. This fixed it. (Most hated phone I ever had btw) Best of luck to you!
Are most of the people you are texting on AT&T network? You may also want to try just dialing *228 + send. Press option 2 instead.