My iPhone 4s won't go into landscape mode when I'm texting!

Try a Reset...

Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.

http://support.apple.com/kb/ht1430

i have this problem too. have tried reset, turning off and back on, and locking/unlocking portrait. still nothing.

mine stays in portrait for all apps and screens, not just texting. so annoying.

It might be locked that way, double click the home button and scroll to the far left.

It is definitely unlocked and as stated in my post, I have tried locking and unlocking the portrait orientation several times, but no luck.

To turn screen rotation on or off, do the following:

1. In iOS 7 or higher, swipe up from the bottom of the screen to reveal Control Center. In iOS 6 or lower,double tap the home button on the front of the iPhone or iPod touch. This will reveal thefast app switcher row of apps at the bottom of the screen
2. In iOS 7, tap the screen rotation lock icon. If it is white, it's on; black and it's off.
3. In iOS 6 or earlier, swipe left to right to reveal the Music app, music player controls, and on the far left, an app with a curved arrow in it (if the lock is turned on, you'll see the lock inside the arrow)
4. To turn screen rotation lock on, tap the curved arrow icon so that the lock appears
5. To turn screen rotation lock off, tap the icon so that the lock disappears
6. When you're done in either version of the iOS, single tap the home button and you'll be returned to normal operation of your device.

Thank You !!!! This works best answer on the internet!!

I swear I don't have the lock/unlock icon. I have all the rest, and yes, I have double tapped home button anc gone to the bottom row. It is not to the left of general, nor to the left of music. Aaaack!