I have the same problem with not being able to text sideways OFTEN. I took it back to Verizon Wireless, only to have it start working. Came home, stopped working. Apple Store, worked fine. Left Apple Store, stopped again. Love the phone but hate that part. My wife has the 6+ and it works fine. If I text portrait style, it works. On notes, emails, searches, sideways typing works fine.............., only texting.
Have only had the phone 3 weeks... Latest IOS as well. Reset, restarted, wiped clean at Verizon, reset, restarted... Still malfunctions.