4S Still ringing after 'Slide to Answer'

For a few months I have been having this problem with my 4S. When I get an incoming call (and the phone is locked), sliding to answer does not work. The slider slides and the red hang up icon displays, but the phone keeps ringing until the other person hangs up or it rings out. I have tried re-setting the phone, reinstalling iOS and everything else I could think of (short of disabling the lock code) and it hasn't worked. This has made me lose some calls from my boss and family, so it is an important issue.

This used to happen about 50% of the time, but is now much more common.

Here is the screen when I slide to answer: http://imgur.com/qv5jg4C