It's always exciting to try to login to My Wallet with GA 2-factor authentication. Half of the times I manage to log in on 3rd attempt, some times after restarting the app on iPhone. Some times I get in from 1st attempt. And other times, like now, my account gets locked for 4 hours after 5 failed attempts.
I had this problem many times. I emailed piuk and reported here... but no response so far. Several other users reported the same problem.
What might help:
1. check server time
http://blockchain.info/time2. adjust phone time to match server time
3. try your luck : )
I do this in case i get error for the first two attempts. It works just fine for me.