I've had this problem on and off for the last few weeks (around, but not directly related, to upgrade to 6.1...can't remember if was before or after)....I'm very convinced it's a software problem (as there have been times when the bottom row works perfectly, without any need to "press" differently or anything).
However, it seems to be a rather resilient software issue, as a full restore does not consistently fix it, and it applies to more than just the keyboard; if I open a drawing app, there's definitely a row (about 4mm up from the edge of the screen closest to home button) where I cannot draw.
It's happened 4 times now. Originally it went away with a restore from backup, then worked for 2 more days. The second and third times, it went away with a full restore (although I did two full restores in between those two restores that did nothing, so restore didn't consistently fix it)....and it seemed to return in the most random of circumstances.
I paid very close attention to what apps I was using the last time it was working so that I could spot which app, if any, might be causing the issue. I deleted several apps, I stopped using the games and many other apps. I only used three apps between it working fine and starting up as a problem again: mail, safari and messages.
I was convinced it was mail, as it happened this morning when I picked up my phone off the bedside table to respond to a new email. It was working when I went to bed. I picked up the phone when it buzzed, hit the email app, hit reply and started to type....**** bottom row wasn't working again.
However, tonight I did 2 things....one of them worked because it went back to normal (sorry, I don't know which as I was exasperated and trying a number of things).
The two things I did between a broken keyboard and a working keyboard were:
1) adding a second keyboard (English(Canada)), changing to this keyboard, and changing back
2) disabling Siri...and then turning it back on...and then disabling Siri's "Raise to Speak"
In retrospect, I wonder if it actually was the "Raise to Speak" that was putting the phone into a weird state, and might explain why the phone would just "suddenly" start behaving strangely when I picked it up. Of note, the "raise to speak" hasn't actually triggered Siri to come on in months anyway.
Regardless, the keyboard is back to normal now...and I'm just going to leave "Raise to Speak" off. I've put the keyboard back to just "English" with no alternates and Siri remains on otherwise without issue.
